        Transition top-bases from old location to new location.

        Beginning with TopGit release 0.19.4, TopGit has the ability to store
        the top-bases refs in either the old `refs/top-bases/...` location or
        the new `refs/heads/{top-bases}/...` location.  Starting with TopGit
        release 0.20.0, the default is the new location.

        By storing the top-bases under heads, Git is less likely to complain
        when manipulating them, hosting providers are more likely to provide
        access to them and Git prevents them from pointing at anything other
        than a commit object.  All in all a win for everyone.

        TopGit attempts to automatically detect whether the new or old location
        is being used for the top-bases and just do the right thing.  However,
        by explicitly setting the config value `topgit.top-bases` to either
        `refs` for the old location or `heads` for the new location the
        auto-detection can be bypassed.  If no top-bases refs are present in
        the repository the default prior to TopGit release 0.20.0 is to use the
        old location but starting with TopGit release 0.20.0 the default is to
        use the new location.

        The `tg migrate-bases` command may be used to migrate top-bases refs
        from the old location to the new location (or, by using the
        undocumented `--reverse` option, vice versa).

        With few exceptions (`tg create -r` and `tg revert`), all top-bases
        refs (both local *and* remote refs) are expected to be stored in the
        same location (either new or old).  A repository's current location for
        storing top-bases refs may be shown with the `tg --top-bases` command.

