Re-enable rev-list --parents for parse_commit which was removed in
(208b2dff95bb48682c351099023a1cbb0e1edf26). rev-list --parents is not
just used to return the parent headers in the commit object, it
includes any grafts which are vaild for the commit.
Signed-off-by: Robert Fitzsimons <robfitz@273k.net> Signed-off-by: Junio C Hamano <junkio@cox.net>