Fix latent costing error in create_merge_append_path.

create_merge_append_path should use the path rowcount it just computed,
not rel->tuples, for costing purposes.  Those numbers should always be
the same at present, but if we ever support parameterized MergeAppend
paths (a case this function is otherwise prepared for), the former would
be right and the latter wrong.

No need for back-patch since the problem is only latent.

Ashutosh Bapat

Discussion: <CAFjFpRek+cLCnTo24youuGtsq4zRphEB8EUUPjDxZjnL4n4HYQ@mail.gmail.com>

src/backend/optimizer/util/pathnode.c

@@ -1333,7 +1333,7 @@ create_merge_append_path(PlannerInfo *root,
 	cost_merge_append(&pathnode->path, root,
 					  pathkeys, list_length(subpaths),
 					  input_startup_cost, input_total_cost,
-					  rel->tuples);
+					  pathnode->path.rows);
 
 	return pathnode;
 }