Improve "nothing to commit" part of status output

Improve "nothing to commit" part of status output

Previously git-status in a clean working directory would advice the user to use
git add. This isn't very helpful when there is nothing to add in the working
directory, therefore note a clean working directory while displaying the other
sections and print the appropriate message for each case.

Signed-off-by: Jürgen Rühle <redacted>
Signed-off-by: Junio C Hamano <redacted>