reftable: change the type of array indices to 'size_t' in reftable/pq.c

reftable: change the type of array indices to 'size_t' in reftable/pq.c

The variables 'i', 'j', 'k' and 'min' are used as indices for
'pq->heap', which is an array. Additionally, 'pq->len' is of
type 'size_t' and is often used to assign values to these
variables. Hence, change the type of these variables from 'int'
to 'size_t'.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>