Fix minor thinko in pathification code.

I passed the wrong "root" struct to create_pathtarget in build_minmax_path.
Since the subroot is a clone of the outer root, this would not cause any
serious problems, but it would waste some cycles because
set_pathtarget_cost_width would not have access to Var width estimates
set up while running query_planner on the subroot.

src/backend/optimizer/plan/planagg.c

@@ -465,7 +465,7 @@ build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
 	 * cheapest path.)
 	 */
 	sorted_path = apply_projection_to_path(subroot, final_rel, sorted_path,
-										   create_pathtarget(root, tlist));
+										   create_pathtarget(subroot, tlist));
 
 	/*
 	 * Determine cost to get just the first row of the presorted path.