Print a MergeJoin's mergeNullsFirst array as bool, not int.

It's declared as being an array of bool, but it's printed
differently from the way bool and arrays of bool are handled
elsewhere.

Patch by Amit Kapila.  Anomaly noted independently by Amit Kapila
and KaiGai Kohei.

src/backend/nodes/outfuncs.c

@@ -649,7 +649,7 @@ _outMergeJoin(StringInfo str, const MergeJoin *node)
 
 	appendStringInfoString(str, " :mergeNullsFirst");
 	for (i = 0; i < numCols; i++)
-		appendStringInfo(str, " %d", (int) node->mergeNullsFirst[i]);
+		appendStringInfo(str, " %s", booltostr(node->mergeNullsFirst[i]));
 }
 
 static void