status: tell the user the working tree is clean

If there is nothing output at all, tell the user the working tree is
completely clean.  It just gives them a bit more of a warm-fuzzy
feeling knowing repo and until the end.  It also more closely
matches with the output of git status.

Signed-off-by: Shawn O. Pearce <sop@google.com>

project.py

@@ -352,7 +352,7 @@ class Project(object):
     df = self.work_git.DiffZ('diff-files')
     do = self.work_git.LsOthers()
     if not di and not df and not do:
-      return
+      return 'CLEAN'
 
     out = StatusColoring(self.config)
     out.project('project %-40s', self.relpath + '/')
@@ -400,6 +400,7 @@ class Project(object):
       else:
         out.write('%s', line)
       out.nl()
+    return 'DIRTY'
 
   def PrintWorkTreeDiff(self):
     """Prints the status of the repository to stdout.