sha1_name: convert get_sha1_mb to struct object_id

All of the callers of this function use struct object_id, so rename it
to get_oid_mb and make it take struct object_id instead of
unsigned char *.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>