merge-ort: unconditionally release attributes index

merge-ort: unconditionally release attributes index

We conditionally release the index used for reading gitattributes in
merge-ort based on whether or the index has been populated. This check
uses `cache_nr` as a condition. This isn't sufficient though, as the
variable may be zero even when some other parts of the index have been
populated. This leads to memory leaks when sparse checkouts are in use,
as we may not end up releasing the sparse checkout patterns.

Fix this issue by unconditionally releasing the index.

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