refs: cleanup directories when deleting packed ref

refs: cleanup directories when deleting packed ref

When deleting a packed ref via 'update-ref -d', a lockfile is made in
the directory that would contain the loose copy of that ref, creating
any directories in the ref's path that do not exist. When the
transaction completes, the lockfile is deleted, but any empty parent
directories made when creating the lockfile are left in place.  These
empty directories are not removed by 'pack-refs' or other housekeeping
tasks and will accumulate over time.

When deleting a loose ref, we remove all empty parent directories at the
end of the transaction.

This commit applies the parent directory cleanup logic used when
deleting loose refs to packed refs as well.

Signed-off-by: Will Chandler <wfc@wfchandler.org>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>