packed_object_info(): use object_id internally for delta base

packed_object_info(): use object_id internally for delta base

The previous commit changed the public interface of packed_object_info()
to return a struct object_id rather than a bare hash. That enables us to
convert our internal helper, as well. We can use nth_packed_object_id()
directly for OFS_DELTA, but we'll still have to use oidread() to pull
the hash for a REF_DELTA out of the packfile.

There should be no additional cost, since we're copying directly into
the object_id the caller provided us (just as we did before; it's just
happening now via nth_packed_object_id()).

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