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/submodule--helper.c

@@ -608,6 +608,7 @@ static void print_status(unsigned int flags, char state, const char *path,
 }
 
 static int handle_submodule_head_ref(const char *refname UNUSED,
+				     const char *referent UNUSED,
 				     const struct object_id *oid,
 				     int flags UNUSED,
 				     void *cb_data)