revert/cherry-pick: work on merge commits as well
Usually you cannot revert a merge because you do not know which
side of the merge should be considered the mainline (iow, what
change to reverse).
With this patch, cherry-pick and revert learn -m (--mainline)
option that lets you specify the parent number (starting from 1)
of the mainline, so that you can:
git revert -m 1 $merge
to reverse the changes introduced by the $merge commit relative
to its first parent, and:
git cherry-pick -m 2 $merge
to replay the changes introduced by the $merge commit relative
to its second parent.
Signed-off-by: Junio C Hamano <gitster@pobox.com>