revision: Fix --topo-order and --max-age with reachability limiting.

What ends up not working very well at all is the combination of
"--topo-order" and the output filter in get_revision. It will
return NULL when we see the first commit out of date-order, even
if we have other commits coming.

So we really should do the "past the date order" thing in
get_revision() only if we have _not_ done it already in
limit_list().

Something like this.

The easiest way to test this is with just

	gitk --since=3.days.ago

on the kernel tree. Without this patch, it tends to be pretty obviously
broken.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>