knock out ReNew if there are no dummies

the operation requires loading the target side, which makes it somewhat
expensive in otherwise unidirectional syncs.

one could also knock out Flags and Delete if there are no living pairs,
but that wouldn't actually save much.

src/sync.c

@@ -680,14 +680,23 @@ box_opened2( sync_vars_t *svars, int t )
 	if (!lock_state( svars ))
 		goto bail;
 
+	int any_dummies[2] = { 0, 0 };
 	int any_purges[2] = { 0, 0 };
 	int any_upgrades[2] = { 0, 0 };
 	int any_new[2] = { 0, 0 };
 	int any_tuids[2] = { 0, 0 };
-	if (svars->replayed) {
+	if (svars->replayed || ((chan->ops[F] | chan->ops[N]) & OP_RENEW)) {
 		for (srec = svars->srecs; srec; srec = srec->next) {
 			if (srec->status & S_DEAD)
 				continue;
+			if (srec->status & S_DUMMY(F))
+				any_dummies[F]++;
+			else if (srec->status & S_DUMMY(N))
+				any_dummies[N]++;
+			else if (srec->status & S_SKIPPED)
+				any_dummies[!srec->uid[F] ? F : N]++;
+			if (!svars->replayed)
+				continue;
 			if (srec->status & S_PURGE) {
 				any_purges[srec->uid[F] ? F : N]++;
 			} else if (srec->status & S_PENDING) {
@@ -721,6 +730,10 @@ box_opened2( sync_vars_t *svars, int t )
 			if (chan->ops[t] & OP_FLAGS)
 				opts[t^1] |= OPEN_FLAGS;
 		}
+		if (!any_dummies[t] && (chan->ops[t] & OP_RENEW)) {
+			chan->ops[t] &= ~OP_RENEW;
+			debug( "no %s dummies; masking ReNew\n", str_fn[t] );
+		}
 		if ((chan->ops[t] & (OP_NEW | OP_RENEW)) || any_new[t] || any_upgrades[t]) {
 			opts[t] |= OPEN_APPEND;
 			if ((chan->ops[t] & OP_NEW) || any_new[t]) {