reftable/stack: refactor function to gather table sizes

reftable/stack: refactor function to gather table sizes

Refactor the function that gathers table sizes to be more idiomatic. For
one, use `REFTABLE_CALLOC_ARRAY()` instead of `reftable_calloc()`.
Second, avoid using an integer to iterate through the tables in the
reftable stack given that `stack_len` itself is using a `size_t`.

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