Fix wrong costing of Sort under Gather Merge.
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: ac34a146981a989f0b6ba30af00feebfd032ca95@mail.gmail.com

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