If you run `tracker index` with no arguments you get the help output.

However, if you ran `tracker index ~/myfile` before you would see this:

    (tracker index:28627): Tracker-WARNING **: (tracker-index.c:369):index_run: code should not be reached

Now you get a more helpful message:

    Use `tracker index --file` when giving a specific file or directory
    to index. See `tracker help index` for more information.

We could probably just assume --file if no mode is passed, but let's do
that separately. To be honest, `tracker index` should probably only take
a file or a mimetype, and the --backup, --restore and --import modes
should become commands in their own right.