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

In 62e745c (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>

Author: pks-t

commit-reach.c

@@ -42,8 +42,7 @@ static int compare_commits_by_gen(const void *_a, const void *_b)
 
 static int queue_has_nonstale(struct prio_queue *queue)
 {
-	int i;
-	for (i = 0; i < queue->nr; i++) {
+	for (size_t i = 0; i < queue->nr; i++) {
 		struct commit *commit = queue->array[i].data;
 		if (!(commit->object.flags & STALE))
 			return 1;