'Tarfile with file that extracts to outside directory.
The path here matches the basename (`self.unsafe_common_dir`), but
then "cd\'s out". E.g. "/usr/../etc" == "/etc", but the naive basename
of the first (but not the second) is "/usr"
Extracting this tarfile in directory <dir> will also put its contents
directly in <dir> (rather than <dir/tarname>).'

def _outside_tar2(self):

    outside_tar = (self.unsafe_common_dir / 'unsafe_file.tar.gz')
    with tarfile.open(outside_tar, 'w:gz') as tar:
        tar.addfile(tarfile.TarInfo(str((self.unsafe_common_dir / '../a_file'))))
    return outside_tar
