ref_transaction_update(): remove "have_old" parameter

Instead, verify the reference's old value if and only if old_sha1 is
non-NULL.

ref_transaction_delete() will get the same treatment in a moment.

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

walker.c

@@ -299,7 +299,7 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 		strbuf_reset(&refname);
 		strbuf_addf(&refname, "refs/%s", write_ref[i]);
 		if (ref_transaction_update(transaction, refname.buf,
-					   &sha1[20 * i], NULL, 0, 0,
+					   &sha1[20 * i], NULL, 0,
 					   msg ? msg : "fetch (unknown)",
 					   &err)) {
 			error("%s", err.buf);