Fix a diff parser issue with a test as well.

parso/grammar.py

@@ -76,8 +76,7 @@ class Grammar(object):
                                 cache_path=cache_path)
                     return module_node
 
-                # TODO I think it's wrong that we have self here.
+                new_node = DiffParser(self._pgen_grammar, module_node).update(
-                new_node = DiffParser(self, module_node).update(
                     old_lines=old_lines,
                     new_lines=lines
                 )

test/test_diff_parser.py

@@ -10,6 +10,15 @@ from parso.python.diff import DiffParser
 from parso.python import parse
 
 
+def test_simple():
+    """
+    The diff parser reuses modules. So check for that.
+    """
+    grammar = load_python_grammar()
+    module_a = grammar.parse('a', diff_cache=True)
+    assert grammar.parse('b', diff_cache=True) == module_a
+
+
 def _check_error_leaves_nodes(node):
     if node.type in ('error_leaf', 'error_node'):
         return True