Make _outPathInfo print the relid set of the path's parent rel.

We can't actually print the parent RelOptInfo in toto, because that would
lead to infinite recursion.  But it's safe enough to reach into the parent
and print its identifying relids, and that makes it a whole lot easier
to figure out what a Path represents.  Should have done this years ago.

src/backend/nodes/outfuncs.c

@@ -1352,12 +1352,15 @@ _outFromExpr(StringInfo str, FromExpr *node)
 /*
  * print the basic stuff of all nodes that inherit from Path
  *
- * Note we do NOT print the parent, else we'd be in infinite recursion
+ * Note we do NOT print the parent, else we'd be in infinite recursion.
+ * We can print the parent's relids for identification purposes, though.
  */
 static void
 _outPathInfo(StringInfo str, Path *node)
 {
 	WRITE_ENUM_FIELD(pathtype, NodeTag);
+	appendStringInfo(str, " :parent_relids ");
+	_outBitmapset(str, node->parent->relids);
 	WRITE_FLOAT_FIELD(startup_cost, "%.2f");
 	WRITE_FLOAT_FIELD(total_cost, "%.2f");
 	WRITE_NODE_FIELD(pathkeys);