dir: fix off by one errors for ignored and untracked entries

dir: fix off by one errors for ignored and untracked entries

In `treat_directory()` we perform some logic to handle ignored and
untracked entries. When populating a directory with entries we first
save the current number of ignored/untracked entries and then populate
new entries at the end of our arrays that keep track of those entries.
When we figure out that all entries have been ignored/are untracked we
then remove this tail of entries from those vectors again. But there is
an off by one error in both paths that causes us to not free the first
ignored and untracked entries, respectively.

Fix these off-by-one errors to plug the resulting leak. While at it,
massage the code a bit to match our modern code style.

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