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

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

--- a/tests/test-py3-commands.t	Thu Jun 01 02:25:18 2017 +0530
+++ b/tests/test-py3-commands.t	Fri Jun 02 10:14:00 2017 +0530
@@ -118,6 +118,13 @@
   $ $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