Re-fix wrong costing of Sort under Gather Merge.

Commit dc02c7bc changed this call
to create_sort_path() to take -1 rather than limit_tuples because,
at that time, there was no way for a Sort beneath a Gather Merge
to become a top-N sort.

Later, commit 3452dc52 provided
a way for a Sort beneath a Gather Merge to become a top-N sort,
but failed to revert the previous commit in the process.  Do that.

Report and analysis by Jeff Janes; patch by Thomas Munro; review by
Amit Kapila and by me.

Discussion: http://postgr.es/m/CAEepm=1BWtC34vUroA0Uqjw02MaqdUrW+d6WD85_k8SLyPiKHQ@mail.gmail.com

src/backend/optimizer/plan/planner.c

@@ -5059,7 +5059,7 @@ create_ordered_paths(PlannerInfo *root,
 											 ordered_rel,
 											 cheapest_partial_path,
 											 root->sort_pathkeys,
-											 -1.0);
+											 limit_tuples);
 
 			total_groups = cheapest_partial_path->rows *
 				cheapest_partial_path->parallel_workers;