Add git-name-rev

Add git-name-rev

git-name-rev tries to find nice symbolic names for commits. It does so by
walking the commits from the refs. When the symbolic name is ambiguous, the
following heuristic is applied: Try to avoid too many ~'s, and if two ambiguous
names have the same count of ~'s, take the one whose last number is smaller.

With "--tags", the names are derived only from tags.

With "--stdin", the stdin is parsed, and after every sha1 for which a name
could be found, the name is appended. (Try "git log | git name-rev --stdin".)

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>