Provide better feedback for the untracked only case in status output

Since 98bf8a47c296f51ea9722fef4bb81dbfb70cd4bb status would claim that
git-commit could be useful even if there are no changes except untracked files.

Since wt-status is already computing all the information needed go the whole
way and actually track the (non-)emptiness of all three sections separately,
unify the code, and provide useful messages for each individual case.

Thanks to Junio and Michael Loeffler for suggestions.

Signed-off-by: Jürgen Rühle <j-r@online.de>