refs: convert dwim_ref and expand_ref to struct object_id

refs: convert dwim_ref and expand_ref to struct object_id

All of the callers of these functions just pass the hash member of a
struct object_id, so convert them to use a pointer to struct object_id
directly.  Insert a check for NULL in expand_ref on a temporary basis;
this check can be removed when resolve_ref_unsafe is converted as well.

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