Make "git checkout" create new branches on demand

Make "git checkout" create new branches on demand

In particular, if we check out something that isn't an old branch, it
now requires a new branch-name to check the thing out into.

So, for example:

	git checkout -b my-branch v2.6.12

will create the new branch "my-branch", and start it at v2.6.12, while

	git checkout master

will just switch back to the master branch.

Of course, if you want to create a new branch "my-branch" and _not_
check it out, you could have done so with just

	git-rev-parse v2.6.12^0 > .git/refs/heads/my-branch

which I think I will codify as "git branch".