reftable/writer: use correct type to iterate through index entries

reftable/writer: use correct type to iterate through index entries

The reftable writer is tracking the number of blocks it has to index via
the `index_len` variable. But while this variable is of type `size_t`,
some sites use an `int` to loop through the index entries.

Convert the code to consistently use `size_t`.

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