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

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>