upload-pack.c: do not pass confusing cb_data to mark_our_ref()

The callee does not use cb_data, and the caller is an intermediate
function in a callchain that later wants to use the cb_data for its
own use.  Clarify the code by breaking the dataflow explicitly by
not passing cb_data down to mark_our_ref().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -742,7 +742,7 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 	const char *refname_nons = strip_namespace(refname);
 	unsigned char peeled[20];
 
-	if (mark_our_ref(refname, sha1, flag, cb_data))
+	if (mark_our_ref(refname, sha1, flag, NULL))
 		return 0;
 
 	if (capabilities)