Fix wrong costing of Sort under Gather Merge.

There's no mechanism for such a sort to become a top-N sort, so we
should pass -1 rather than limit_tuples to cost_sort().

Rushabh Lathia, per a report from Mithun Cy

Discussion: http://postgr.es/m/CAGPqQf1akRcSgC9=6iwx=sEPap9UvPpHJLzg8_N+OuHdb6fL+g@mail.gmail.com

src/backend/optimizer/plan/planner.c

@@ -4416,7 +4416,7 @@ create_ordered_paths(PlannerInfo *root,
 											 ordered_rel,
 											 cheapest_partial_path,
 											 root->sort_pathkeys,
-											 limit_tuples);
+											 -1.0);
 
 			total_groups = cheapest_partial_path->rows *
 				cheapest_partial_path->parallel_workers;