patch-ids: replace the seen indicator with a commit pointer

patch-ids: replace the seen indicator with a commit pointer

The cherry_pick_list was looping through the original side checking the
seen indicator and setting the cherry_flag on the commit.  If we save
off the commit in the patch_id we can set the cherry_flag on the correct
commit when running through the other side when a patch_id match is found.

Signed-off-by: Kevin Willford <kcwillford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>