Add support for listing untracked files too

It would be nice to be able to list untracked files in this view as well.

Use case:
- I create a new file in my repo but I haven't added it yet (I do this at the end of my workflow)
- I edit the file a bunch and save it
- I come back to emacs later and try to open it using `helm-browse-project` and can't find it