List all available package upgrades.

    refresh
        Whether or not to sync the portage tree before checking for upgrades.

    backtrack
        Specifies an integer number of times to backtrack if dependency
        calculation fails due to a conflict or an unsatisfied dependency
        (default: ´3´).

        .. versionadded: 2015.8.0

    CLI Example:

    .. code-block:: bash

        salt '*' pkg.list_upgrades

def list_upgrades(refresh=True, backtrack=3, **kwargs):  # pylint: disable=W0613
    '''
    List all available package upgrades.

    refresh
        Whether or not to sync the portage tree before checking for upgrades.

    backtrack
        Specifies an integer number of times to backtrack if dependency
        calculation fails due to a conflict or an unsatisfied dependency
        (default: ´3´).

        .. versionadded: 2015.8.0

    CLI Example:

    .. code-block:: bash

        salt '*' pkg.list_upgrades
    '''
    if salt.utils.data.is_true(refresh):
        refresh_db()
    return _get_upgradable(backtrack)