Fix to display the usage message of the command download when the user

don't provide any arguments to 'repo download'.

Signed-off-by: Thiago Farina <thiago.farina@gmail.com>

subcmds/download.py

@@ -36,6 +36,9 @@ makes it available in your project's local working directory.
     pass
 
   def _ParseChangeIds(self, args):
+    if not args:
+      self.Usage()
+
     to_get = []
     project = None