        Change the dependencies of a TopGit-controlled topic branch.
        This should have several subcommands, but only ``add`` is
        supported right now.

        The ``add`` subcommand takes an argument naming a topic branch to
        be added, adds it to ``.topdeps``, performs a commit and then
        updates your topic branch accordingly.  If you want to do other
        things related to the dependency addition, like adjusting
        ``.topmsg``, use the option ``--no-commit``.  Adding the
        ``--no-update`` (or ``--no-commit``) option will suppress the
        ``tg update`` normally performed after committing the change.

        It is safe to run ``tg depend add`` in a dirty worktree, but the
        normally performed ``tg update`` will be suppressed in that case
        (even if neither ``--no-update`` nor ``--no-commit`` is given).

        You have enabled ``git rerere`` haven't you?

        | TODO: Subcommand for removing dependencies, obviously

