reftable/writer: fix type used for number of records

reftable/writer: fix type used for number of records

Both `reftable_writer_add_refs()` and `reftable_writer_add_logs()`
accept an array of records that should be added to the new table.
Callers of this function are expected to also pass the number of such
records to the function to tell it how many such records it is supposed
to write.

But while all callers pass in a `size_t`, which is a sensible choice,
the function in fact accepts an `int` as argument, which is less so. Fix
this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>