[PATCH] Add 'git show-branch'.

[PATCH] Add 'git show-branch'.

The 'git show-branches' command turns out to be reasonably useful,
but painfully slow.  So rewrite it in C, using ideas from merge-base
while enhancing it a bit more.

 - Unlike show-branches, it can take --heads (show me all my
   heads), --tags (show me all my tags), or --all (both).

 - It can take --more=<number> to show beyond the merge-base.

 - It shows the short name for each commit in the extended SHA1
   syntax.

 - It can find merge-base for more than two heads.

Examples:

    $ git show-branch --more=6 HEAD

    is almost the same as "git log --pretty=oneline --max-count=6".

    $ git show-branch --merge-base master mhf misc

    finds the merge base of the three given heads.

    $ git show-branch master mhf misc

    shows logs from the top of these three branch heads, up to their
    common ancestor commit is shown.

    $ git show-branch --all --more=10

    is poor-man's gitk, showing all the tags and heads, and
    going back 10 commits beyond the merge base of those refs.

Signed-off-by: Junio C Hamano <junkio@cox.net>