sep, generate all elements of that path

    >>> list(_ancestry('b/d'))
    ['b/d', 'b']
    >>> list(_ancestry('/b/d/'))
    ['/b/d', '/b']
    >>> list(_ancestry('b/d/f/'))
    ['b/d/f', 'b/d', 'b']
    >>> list(_ancestry('b'))
    ['b']
    >>> list(_ancestry(''))
    []
    N)