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>