reftable/stack: index segments with `size_t`

reftable/stack: index segments with `size_t`

We use `int`s to index into arrays of segments and track the length of
them, which is considered to be a code smell in the Git project. Convert
the code to use `size_t` instead.

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