rev-list --boundary: show boundary commits even when limited otherwise.

rev-list --boundary: show boundary commits even when limited otherwise.

The boundary commits are shown for UI like gitk to draw them as
soon as topo-order sorting allows, and should not be omitted by
get_revision() filtering logic.  As long as their immediate
child commits are shown, we should not filter them out.

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