ref-filter: use "struct object_id" consistently

ref-filter: use "struct object_id" consistently

Internally we store a "struct object_id", and all of our
callers have one to pass us. But we insist that they peel it
to its bare-sha1 hash, which we then hashcpy() into place.
Let's pass it around as an object_id, which future-proofs us
for a post-sha1 world.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>