Re-fix wrong costing of Sort under Gather Merge.

Re-fix wrong costing of Sort under Gather Merge.

Commit dc02c7bca4dccf7de278cdc6b3325a829e75b252 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 3452dc5240da43e833118484e1e9b4894d04431c 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