Given the hanoi_rotator generated files in the output directory,
    returns the rotation_id that will be given to the current file. If there
    are no existing rotated files, return 0.

def _next_rotation_id(rotated_files):
    """Given the hanoi_rotator generated files in the output directory,
    returns the rotation_id that will be given to the current file. If there
    are no existing rotated files, return 0.
    """
    if not rotated_files:
        return 0
    else:
        highest_rotated_file = max(rotated_files, key=lambda x: x[1])
        return highest_rotated_file[1] + 1