refs: convert update_ref and refs_update_ref to use struct object_id

refs: convert update_ref and refs_update_ref to use struct object_id

Convert update_ref, refs_update_ref, and write_pseudoref to use struct
object_id.  Update the existing callers as well.  Remove update_ref_oid,
as it is no longer needed.

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