dir: release untracked cache data

dir: release untracked cache data

There are several cases where we invalidate untracked cache directory
entries where we do not free the underlying data, but reset the number
of entries. This causes us to leak memory because `free_untracked()`
will not iterate over any potential entries which we still had in the
array.

Fix this issue by freeing old entries. The leak is exposed by t7519, but
plugging it alone does not make the whole test suite pass.

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