commit-reach: fix index used to loop through unsigned integer

commit-reach: fix index used to loop through unsigned integer

In 62e745ced2 (prio-queue: use size_t rather than int for size,
2024-12-20), we refactored `struct prio_queue` to track the number of
contained entries via a `size_t`. While the refactoring adapted one of
the users of that variable, it forgot to also adapt "commit-reach.c"
accordingly. This was missed because that file has -Wsign-conversion
disabled.

Fix the issue by using a `size_t` to iterate through entries.

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