rev-list --max-age, --max-count: support --boundary

Now, when saying --max-age=<timestamp>, or --max-count=<n>, together
with --boundary, rev-list prints the boundary commits, i.e. the
commits which are _just_ not shown without --boundary, i.e. their
children are, but they aren't.

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