+How does the merge work?
+------------------------
+
+We said this tutorial shows what plumbing does to help you cope
+with the porcelain that isn't flushing, but we so far did not
+talk about how the merge really works. If you are following
+this tutorial the first time, I'd suggest to skip to "Publishing
+your work" section and come back here later.
+
+OK, still with me? To give us an example to look at, let's go
+back to the earlier repository with "hello" and "example" file,
+and bring ourselves back to the pre-merge state:
+
+------------
+$ git show-branch --more=3 master mybranch
+! [master] Merge work in mybranch
+ * [mybranch] Merge work in mybranch
+--
+++ [master] Merge work in mybranch
+++ [master^2] Some work.
+++ [master^] Some fun.
+------------
+
+Remember, before running `git merge`, our `master` head was at
+"Some fun." commit, while our `mybranch` head was at "Some
+work." commit.
+
+------------
+$ git checkout mybranch
+$ git reset --hard master^2
+$ git checkout master
+$ git reset --hard master^
+------------
+
+After rewinding, the commit structure should look like this:
+
+------------
+$ git show-branch
+* [master] Some fun.
+ ! [mybranch] Some work.
+--
+ + [mybranch] Some work.
++ [master] Some fun.
+++ [mybranch^] New day.
+------------
+
+Now we are ready to experiment with the merge by hand.
+
+`git merge` command, when merging two branches, uses 3-way merge
+algorithm. First, it finds the common ancestor between them.
+The command it uses is `git-merge-base`:
+
+------------
+$ mb=$(git-merge-base HEAD mybranch)
+------------
+
+The command writes the commit object name of the common ancestor
+to the standard output, so we captured its output to a variable,
+because we will be using it in the next step. BTW, the common
+ancestor commit is the "New day." commit in this case. You can
+tell it by:
+
+------------
+$ git-name-rev $mb
+my-first-tag
+------------
+
+After finding out a common ancestor commit, the second step is
+this:
+
+------------
+$ git-read-tree -m -u $mb HEAD mybranch
+------------
+
+This is the same `git-read-tree` command we have already seen,
+but it takes three trees, unlike previous examples. This reads
+the contents of each tree into different 'stage' in the index
+file (the first tree goes to stage 1, the second stage 2,
+etc.). After reading three trees into three stages, the paths
+that are the same in all three stages are 'collapsed' into stage
+0. Also paths that are the same in two of three stages are
+collapsed into stage 0, taking the SHA1 from either stage 2 or
+stage 3, whichever is different from stage 1 (i.e. only one side
+changed from the common ancestor).
+
+After 'collapsing' operation, paths that are different in three
+trees are left in non-zero stages. At this point, you can
+inspect the index file with this command:
+
+------------
+$ git-ls-files --stage
+100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example
+100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello
+100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello
+100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
+------------
+
+In our example of only two files, we did not have unchanged
+files so only 'example' resulted in collapsing, but in real-life
+large projects, only small number of files change in one commit,
+and this 'collapsing' tends to trivially merge most of the paths
+fairly quickly, leaving only a handful the real changes in non-zero
+stages.
+
+To look at only non-zero stages, use `\--unmerged` flag:
+
+------------
+$ git-ls-files --unmerged
+100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello
+100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello
+100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
+------------
+
+The next step of merging is to merge these three versions of the
+file, using 3-way merge. This is done by giving
+`git-merge-one-file` command as one of the arguments to
+`git-merge-index` command:
+
+------------
+$ git-merge-index git-merge-one-file hello
+Auto-merging hello.
+merge: warning: conflicts during merge
+ERROR: Merge conflict in hello.
+fatal: merge program failed
+------------
+
+`git-merge-one-file` script is called with parameters to
+describe those three versions, and is responsible to leave the
+merge results in the working tree and register it in the index
+file. It is a fairly straightforward shell script, and
+eventually calls `merge` program from RCS suite to perform the
+file-level 3-way merge. In this case, `merge` detects
+conflicts, and the merge result with conflict marks is left in
+the working tree, while the index file is updated with the
+version from the current branch (this is to make `git diff`
+useful after this step). This can be seen if you run `ls-files
+--stage` again at this point:
+
+------------
+$ git-ls-files --stage
+100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example
+100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 0 hello
+------------
+
+As you can see, there is no unmerged paths in the index file.
+This is the state of the index file and the working file after
+`git merge` returns control back to you, leaving the conflicting
+merge for you to resolve.
+
+
+Publishing your work
+--------------------
+
+So we can use somebody else's work from a remote repository; but
+how can *you* prepare a repository to let other people pull from
+it?
+
+Your do your real work in your working tree that has your
+primary repository hanging under it as its `.git` subdirectory.
+You *could* make that repository accessible remotely and ask
+people to pull from it, but in practice that is not the way
+things are usually done. A recommended way is to have a public
+repository, make it reachable by other people, and when the
+changes you made in your primary working tree are in good shape,
+update the public repository from it. This is often called
+'pushing'.
+
+[NOTE]
+This public repository could further be mirrored, and that is
+how git repositories at `kernel.org` are managed.
+
+Publishing the changes from your local (private) repository to
+your remote (public) repository requires a write privilege on
+the remote machine. You need to have an SSH account there to
+run a single command, `git-receive-pack`.
+
+First, you need to create an empty repository on the remote
+machine that will house your public repository. This empty
+repository will be populated and be kept up-to-date by pushing
+into it later. Obviously, this repository creation needs to be
+done only once.
+
+[NOTE]
+`git push` uses a pair of programs,
+`git-send-pack` on your local machine, and `git-receive-pack`
+on the remote machine. The communication between the two over
+the network internally uses an SSH connection.
+
+Your private repository's git directory is usually `.git`, but
+your public repository is often named after the project name,
+i.e. `<project>.git`. Let's create such a public repository for
+project `my-git`. After logging into the remote machine, create
+an empty directory:
+
+------------
+$ mkdir my-git.git
+------------
+
+Then, make that directory into a git repository by running
+`git init-db`, but this time, since its name is not the usual
+`.git`, we do things slightly differently:
+
+------------
+$ GIT_DIR=my-git.git git-init-db
+------------
+
+Make sure this directory is available for others you want your
+changes to be pulled by via the transport of your choice. Also
+you need to make sure that you have the `git-receive-pack`
+program on the `$PATH`.
+
+[NOTE]
+Many installations of sshd do not invoke your shell as the login
+shell when you directly run programs; what this means is that if
+your login shell is `bash`, only `.bashrc` is read and not
+`.bash_profile`. As a workaround, make sure `.bashrc` sets up
+`$PATH` so that you can run `git-receive-pack` program.
+
+[NOTE]
+If you plan to publish this repository to be accessed over http,
+you should do `chmod +x my-git.git/hooks/post-update` at this
+point. This makes sure that every time you push into this
+repository, `git-update-server-info` is run.
+
+Your "public repository" is now ready to accept your changes.
+Come back to the machine you have your private repository. From
+there, run this command:
+
+------------
+$ git push <public-host>:/path/to/my-git.git master
+------------
+
+This synchronizes your public repository to match the named
+branch head (i.e. `master` in this case) and objects reachable
+from them in your current repository.
+
+As a real example, this is how I update my public git
+repository. Kernel.org mirror network takes care of the
+propagation to other publicly visible machines:
+
+------------
+$ git push master.kernel.org:/pub/scm/git/git.git/
+------------
+
+
+Packing your repository
+-----------------------
+
+Earlier, we saw that one file under `.git/objects/??/` directory
+is stored for each git object you create. This representation
+is efficient to create atomically and safely, but
+not so convenient to transport over the network. Since git objects are
+immutable once they are created, there is a way to optimize the
+storage by "packing them together". The command
+
+------------
+$ git repack
+------------
+
+will do it for you. If you followed the tutorial examples, you
+would have accumulated about 17 objects in `.git/objects/??/`
+directories by now. `git repack` tells you how many objects it
+packed, and stores the packed file in `.git/objects/pack`
+directory.
+
+[NOTE]
+You will see two files, `pack-\*.pack` and `pack-\*.idx`,
+in `.git/objects/pack` directory. They are closely related to
+each other, and if you ever copy them by hand to a different
+repository for whatever reason, you should make sure you copy
+them together. The former holds all the data from the objects
+in the pack, and the latter holds the index for random
+access.
+
+If you are paranoid, running `git-verify-pack` command would
+detect if you have a corrupt pack, but do not worry too much.
+Our programs are always perfect ;-).
+
+Once you have packed objects, you do not need to leave the
+unpacked objects that are contained in the pack file anymore.
+
+------------
+$ git prune-packed
+------------
+
+would remove them for you.
+
+You can try running `find .git/objects -type f` before and after
+you run `git prune-packed` if you are curious. Also `git
+count-objects` would tell you how many unpacked objects are in
+your repository and how much space they are consuming.
+
+[NOTE]
+`git pull` is slightly cumbersome for HTTP transport, as a
+packed repository may contain relatively few objects in a
+relatively large pack. If you expect many HTTP pulls from your
+public repository you might want to repack & prune often, or
+never.
+
+If you run `git repack` again at this point, it will say
+"Nothing to pack". Once you continue your development and
+accumulate the changes, running `git repack` again will create a
+new pack, that contains objects created since you packed your
+repository the last time. We recommend that you pack your project
+soon after the initial import (unless you are starting your
+project from scratch), and then run `git repack` every once in a
+while, depending on how active your project is.
+
+When a repository is synchronized via `git push` and `git pull`
+objects packed in the source repository are usually stored
+unpacked in the destination, unless rsync transport is used.
+While this allows you to use different packing strategies on
+both ends, it also means you may need to repack both
+repositories every once in a while.
+
+
+Working with Others
+-------------------
+
+Although git is a truly distributed system, it is often
+convenient to organize your project with an informal hierarchy
+of developers. Linux kernel development is run this way. There
+is a nice illustration (page 17, "Merges to Mainline") in Randy
+Dunlap's presentation (`http://tinyurl.com/a2jdg`).
+
+It should be stressed that this hierarchy is purely *informal*.
+There is nothing fundamental in git that enforces the "chain of
+patch flow" this hierarchy implies. You do not have to pull
+from only one remote repository.
+
+A recommended workflow for a "project lead" goes like this:
+
+1. Prepare your primary repository on your local machine. Your
+ work is done there.
+
+2. Prepare a public repository accessible to others.
++
+If other people are pulling from your repository over dumb
+transport protocols, you need to keep this repository 'dumb
+transport friendly'. After `git init-db`,
+`$GIT_DIR/hooks/post-update` copied from the standard templates
+would contain a call to `git-update-server-info` but the
+`post-update` hook itself is disabled by default -- enable it
+with `chmod +x post-update`.
+
+3. Push into the public repository from your primary
+ repository.
+
+4. `git repack` the public repository. This establishes a big
+ pack that contains the initial set of objects as the
+ baseline, and possibly `git prune` if the transport
+ used for pulling from your repository supports packed
+ repositories.
+
+5. Keep working in your primary repository. Your changes
+ include modifications of your own, patches you receive via
+ e-mails, and merges resulting from pulling the "public"
+ repositories of your "subsystem maintainers".
++
+You can repack this private repository whenever you feel like.
+
+6. Push your changes to the public repository, and announce it
+ to the public.
+
+7. Every once in a while, "git repack" the public repository.
+ Go back to step 5. and continue working.
+
+
+A recommended work cycle for a "subsystem maintainer" who works
+on that project and has an own "public repository" goes like this:
+
+1. Prepare your work repository, by `git clone` the public
+ repository of the "project lead". The URL used for the
+ initial cloning is stored in `.git/remotes/origin`.
+
+2. Prepare a public repository accessible to others, just like
+ the "project lead" person does.
+
+3. Copy over the packed files from "project lead" public
+ repository to your public repository, unless the "project
+ lead" repository lives on the same machine as yours. In the
+ latter case, you can use `objects/info/alternates` file to
+ point at the repository you are borrowing from.
+
+4. Push into the public repository from your primary
+ repository. Run `git repack`, and possibly `git prune` if the
+ transport used for pulling from your repository supports
+ packed repositories.
+
+5. Keep working in your primary repository. Your changes
+ include modifications of your own, patches you receive via
+ e-mails, and merges resulting from pulling the "public"
+ repositories of your "project lead" and possibly your
+ "sub-subsystem maintainers".
++
+You can repack this private repository whenever you feel
+like.
+
+6. Push your changes to your public repository, and ask your
+ "project lead" and possibly your "sub-subsystem
+ maintainers" to pull from it.
+
+7. Every once in a while, `git repack` the public repository.
+ Go back to step 5. and continue working.
+
+
+A recommended work cycle for an "individual developer" who does
+not have a "public" repository is somewhat different. It goes
+like this:
+
+1. Prepare your work repository, by `git clone` the public
+ repository of the "project lead" (or a "subsystem
+ maintainer", if you work on a subsystem). The URL used for
+ the initial cloning is stored in `.git/remotes/origin`.
+
+2. Do your work in your repository on 'master' branch.
+
+3. Run `git fetch origin` from the public repository of your
+ upstream every once in a while. This does only the first
+ half of `git pull` but does not merge. The head of the
+ public repository is stored in `.git/refs/heads/origin`.
+
+4. Use `git cherry origin` to see which ones of your patches
+ were accepted, and/or use `git rebase origin` to port your
+ unmerged changes forward to the updated upstream.
+
+5. Use `git format-patch origin` to prepare patches for e-mail
+ submission to your upstream and send it out. Go back to
+ step 2. and continue.
+
+
+Working with Others, Shared Repository Style
+--------------------------------------------
+
+If you are coming from CVS background, the style of cooperation
+suggested in the previous section may be new to you. You do not
+have to worry. git supports "shared public repository" style of
+cooperation you are probably more familiar with as well.
+
+For this, set up a public repository on a machine that is
+reachable via SSH by people with "commit privileges". Put the
+committers in the same user group and make the repository
+writable by that group.
+
+You, as an individual committer, then:
+
+- First clone the shared repository to a local repository:
+------------------------------------------------
+$ git clone repo.shared.xz:/pub/scm/project.git/ my-project
+$ cd my-project
+$ hack away
+------------------------------------------------
+
+- Merge the work others might have done while you were hacking
+ away:
+------------------------------------------------
+$ git pull origin
+$ test the merge result
+------------------------------------------------
+[NOTE]
+================================
+The first `git clone` would have placed the following in
+`my-project/.git/remotes/origin` file, and that's why this and
+the next step work.
+------------
+URL: repo.shared.xz:/pub/scm/project.git/ my-project
+Pull: master:origin
+------------
+================================
+
+- push your work as the new head of the shared
+ repository.
+------------------------------------------------
+$ git push origin master
+------------------------------------------------
+If somebody else pushed into the same shared repository while
+you were working locally, `git push` in the last step would
+complain, telling you that the remote `master` head does not
+fast forward. You need to pull and merge those other changes
+back before you push your work when it happens.
+
+
+Bundling your work together
+---------------------------
+
+It is likely that you will be working on more than one thing at
+a time. It is easy to use those more-or-less independent tasks
+using branches with git.
+
+We have already seen how branches work in a previous example,
+with "fun and work" example using two branches. The idea is the
+same if there are more than two branches. Let's say you started
+out from "master" head, and have some new code in the "master"
+branch, and two independent fixes in the "commit-fix" and
+"diff-fix" branches:
+
+------------
+$ git show-branch
+! [commit-fix] Fix commit message normalization.
+ ! [diff-fix] Fix rename detection.
+ * [master] Release candidate #1
+---
+ + [diff-fix] Fix rename detection.
+ + [diff-fix~1] Better common substring algorithm.
++ [commit-fix] Fix commit message normalization.
+ + [master] Release candidate #1
++++ [diff-fix~2] Pretty-print messages.
+------------
+
+Both fixes are tested well, and at this point, you want to merge
+in both of them. You could merge in 'diff-fix' first and then
+'commit-fix' next, like this:
+
+------------
+$ git merge 'Merge fix in diff-fix' master diff-fix
+$ git merge 'Merge fix in commit-fix' master commit-fix
+------------
+
+Which would result in:
+
+------------
+$ git show-branch
+! [commit-fix] Fix commit message normalization.
+ ! [diff-fix] Fix rename detection.
+ * [master] Merge fix in commit-fix
+---
+ + [master] Merge fix in commit-fix
++ + [commit-fix] Fix commit message normalization.
+ + [master~1] Merge fix in diff-fix
+ ++ [diff-fix] Fix rename detection.
+ ++ [diff-fix~1] Better common substring algorithm.
+ + [master~2] Release candidate #1
++++ [master~3] Pretty-print messages.
+------------
+
+However, there is no particular reason to merge in one branch
+first and the other next, when what you have are a set of truly
+independent changes (if the order mattered, then they are not
+independent by definition). You could instead merge those two
+branches into the current branch at once. First let's undo what
+we just did and start over. We would want to get the master
+branch before these two merges by resetting it to 'master~2':
+
+------------
+$ git reset --hard master~2
+------------
+
+You can make sure 'git show-branch' matches the state before
+those two 'git merge' you just did. Then, instead of running
+two 'git merge' commands in a row, you would pull these two
+branch heads (this is known as 'making an Octopus'):
+
+------------
+$ git pull . commit-fix diff-fix
+$ git show-branch
+! [commit-fix] Fix commit message normalization.
+ ! [diff-fix] Fix rename detection.
+ * [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
+---
+ + [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
++ + [commit-fix] Fix commit message normalization.
+ ++ [diff-fix] Fix rename detection.
+ ++ [diff-fix~1] Better common substring algorithm.
+ + [master~1] Release candidate #1
++++ [master~2] Pretty-print messages.
+------------