Make an old mock.patch.dict on os.environ clearer

This clarifies that the object's contents are patched, rather than
patching the attribute used to get the object in the first place.
It is also in keeping with the style of patching os.environ
elsewhere in the test suite.

Author: EliahKagan

test/test_git.py

@@ -245,7 +245,7 @@ def test_insert_after_kwarg_raises(self):
 
     def test_env_vars_passed_to_git(self):
         editor = "non_existent_editor"
-        with mock.patch.dict("os.environ", {"GIT_EDITOR": editor}):  # @UndefinedVariable
+        with mock.patch.dict(os.environ, {"GIT_EDITOR": editor}):
             self.assertEqual(self.git.var("GIT_EDITOR"), editor)
 
     @with_rw_directory