ref_transaction_delete(): check that old_sha1 is not null_sha1

It makes no sense to delete a reference that is already known not to
exist.

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>

refs.c

@@ -3702,6 +3702,8 @@ int ref_transaction_delete(struct ref_transaction *transaction,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
+	if (old_sha1 && is_null_sha1(old_sha1))
+		die("BUG: delete called with old_sha1 set to zeros");
 	return ref_transaction_update(transaction, refname,
 				      null_sha1, old_sha1,
 				      flags, msg, err);