Change the signature of read_packed_refs()

Change it to return a (struct ref_list *) instead of writing into
a cached_refs structure.  (This removes the need to create a
cached_refs structure in resolve_gitlink_packed_ref(), where it
is otherwise unneeded.)

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>