commit-reach: use `size_t` to track indices in `get_reachable_subset()`

commit-reach: use `size_t` to track indices in `get_reachable_subset()`

Similar as with the preceding commit, adapt `get_reachable_subset()` so
that it tracks array indices via `size_t` instead of using signed
integers to fix a couple of -Wsign-compare warnings. Adapt callers
accordingly.

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