reftable/pq: use `size_t` to track iterator index

The reftable priority queue is used by the merged iterator to yield records from its sub-iterators in the expected order. Each entry has a record corresponding to such a sub-iterator as well as an index that indicates which sub-iterator the record belongs to. But while the sub-iterators are tracked with a `size_t`, we store the index as an `int` in the entry. Fix this and use `size_t` consistently. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>