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>

reftable/pq.h

@@ -12,7 +12,7 @@ https://developers.google.com/open-source/licenses/bsd
 #include "record.h"
 
 struct pq_entry {
-	int index;
+	size_t index;
 	struct reftable_record rec;
 };