refs_read_ref_full() wraps refs_resolve_ref_unsafe(), which handles a
NULL oid pointer of callers not interested in the resolved object ID.
Pass NULL from files_copy_or_rename_ref() to clarify that it is one
such caller.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>