Fix EquivalenceClass processing for nested append relations.

The original coding of EquivalenceClasses didn't foresee that appendrel
child relations might themselves be appendrels; but this is possible for
example when a UNION ALL subquery scans a table with inheritance children.
The oversight led to failure to optimize ordering-related issues very well
for the grandchild tables.  After some false starts involving explicitly
flattening the appendrel representation, we found that this could be fixed
easily by removing a few implicit assumptions about appendrel parent rels
not being children themselves.

Kyotaro Horiguchi and Tom Lane, reviewed by Noah Misch

src/backend/optimizer/plan/createplan.c

@@ -751,7 +751,7 @@ create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path)
 
 	/* Compute sort column info, and adjust MergeAppend's tlist as needed */
 	(void) prepare_sort_from_pathkeys(root, plan, pathkeys,
-									  NULL,
+									  best_path->path.parent->relids,
 									  NULL,
 									  true,
 									  &node->numCols,