rebase -i: use struct object_id when writing state

rebase -i: use struct object_id when writing state

Rather than passing a string around pass the struct object_id that the
string was created from call oid_hex() when we write the file.

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