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 <j-r@online.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

wt-status.h

@@ -16,6 +16,7 @@ struct wt_status {
 	int verbose;
 	int amend;
 	int untracked;
+	int workdir_clean;
 };
 
 int git_status_config(const char *var, const char *value);