refs: add referent to each_ref_fn

Add a parameter to each_ref_fn so that callers to the ref APIs
that use this function as a callback can have acess to the
unresolved value of a symbolic ref.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/repack.c

@@ -667,6 +667,7 @@ struct midx_snapshot_ref_data {
 };
 
 static int midx_snapshot_ref_one(const char *refname UNUSED,
+				 const char *referent UNUSED,
 				 const struct object_id *oid,
 				 int flag UNUSED, void *_data)
 {