Built-in cherry

This replaces the shell script git-cherry with a version written in C.

The behaviour of the new version differs from the original in two
points: it has no long help any more, and it is handling the (optional)
third parameter a bit differently.  Basically, it does the equivalent
of

   ours=`git-rev-list $ours ^$limit ^$upstream`

instead of

   ours=`git-rev-list $ours ^$limit`

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>