py3: add a test to show `hg diff` works on Python 3

tests/test-py3-commands.t

@@ -118,6 +118,13 Test bytes-ness of policy.policy with HG
   $ $PYTHON3 $HGBIN add iota
   $ $PYTHON3 $HGBIN status
   A iota
+  $ hg diff --nodates --git
+  diff --git a/iota b/iota
+  new file mode 100644
+  --- /dev/null
+  +++ b/iota
+  @@ -0,0 +1,1 @@
+  +This is the file 'iota'.
   $ $PYTHON3 $HGBIN commit --message 'commit performed in Python 3'
   $ $PYTHON3 $HGBIN status