rebase -i: use struct object_id rather than looking up commit

rebase -i: use struct object_id rather than looking up commit

We already have a struct object_id containing the oid that we want to
set ORIG_HEAD to so use that rather than converting it to a string and
then calling get_oid() on that string.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>