Show help when cd is not given an argument

src/py65/monitor.py

@@ -457,6 +457,9 @@ class Monitor(cmd.Cmd):
         self._output("Change the working directory.")
 
     def do_cd(self, args):
+        if args == '':
+            return self.help_cd()
+
         try:
             os.chdir(args)
         except OSError, why:

src/py65/tests/test_monitor.py

@@ -149,6 +149,14 @@ class MonitorTests(unittest.TestCase):
         out = stdout.getvalue()
         self.assertTrue(out.startswith("cd <directory>"))
 
+    def test_do_cd_with_no_dir_shows_help(self):
+        stdout = StringIO()
+        mon = Monitor(stdout=stdout)
+        mon.do_cd("")
+
+        out = stdout.getvalue()
+        self.assertTrue(out.startswith("cd <directory>"))
+
     # delete_label
 
     def test_shortcut_for_delete_label(self):