Don't do anything with the diff parser if nothing changes.

test/test_parser/test_diff_parser.py

@@ -420,7 +420,7 @@ def test_whitespace_at_end(differ):
     code = dedent('str\n\n')
 
     differ.initialize(code)
-    differ.parse(code, parsers=1, copies=1)
+    differ.parse(code + '\n', parsers=1, copies=1)
 
 
 def test_in_class_movements(differ):