Teach rev-list since..til notation.

The King Penguin says:

Now, for extra bonus points, maybe you should make "git-rev-list" also
understand the "rev..rev" format (which you can't do with just the
get_sha1() interface, since it expands into more).

The faithful servant makes it so.

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