Show usage when no subsubcommand was given.

DepCommand.py

@@ -28,7 +28,11 @@ class DepCommand(Command):
                  p_err=lambda a: None,
                  p_prompt=lambda a: None):
         super(DepCommand, self).__init__(p_args, p_todolist, p_out, p_err, p_prompt)
-        self.subsubcommand = self.argument(0)
+
+        try:
+            self.subsubcommand = self.argument(0)
+        except InvalidCommandArgument:
+            self.subsubcommand = None
 
     def _handle_add(self):
         (from_todo, to_todo) = self._get_todos()

test/DepCommandTest.py

@@ -194,3 +194,11 @@ class DepCommandTest(CommandTest.CommandTest):
         self.assertEqual(self.errors, command.usage() + "\n")
         self.assertFalse(self.todolist.is_dirty())
 
+    def test_no_subsubcommand(self):
+        command = DepCommand.DepCommand([], self.todolist, self.out, self.error)
+        command.execute()
+
+        self.assertFalse(self.output)
+        self.assertEqual(self.errors, command.usage() + "\n")
+        self.assertFalse(self.todolist.is_dirty())
+