ion. Valid values are ``abort``, ``none``,
    ``linear``, and ``noconflict``. ``abort`` always fails if the working
    directory has uncommitted changes. ``none`` performs no checking, and may
    result in a merge with uncommitted changes. ``linear`` allows any update
    as long as it follows a straight line in the revision history, and may
    trigger a merge with uncommitted changes. ``noconflict`` will allow any
    update which would not trigger a merge with uncommitted changes, if any
    are present.
    (default: ``linear``)