bisect: fix off-by-one error in `best_bisection_sorted()`

bisect: fix off-by-one error in `best_bisection_sorted()`

After we have sorted the `cnt`-many commits that we have selected, we
place them into the commit list. We then set `p->next` to NULL, but as
we do so, `p` is already pointing one beyond item number `cnt`. Indeed,
we check whether `p` is NULL before dereferencing it.

This only matters if there are TREESAME-commits. Since they should be
skipped, they are not included in `cnt` and we will hit the situation
where we set `p->next` to NULL. As a result, the list will be one longer
than it should be. The last commit in the list will be one which occurs
earlier, or which shouldn't be included.

Do not update `p` the very last round in the loop. This ensures that
after the loop, `p->next` points to the remainder of the list, and we
can set it to NULL. While we're here, free that remainder to fix a
memory leak.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>