Fix CliRunnerCallbacks.on_file_diff in callbacks.py

When running ansible -C, I get a traceback that ends with
    AttributeError: Values instance has no attribute 'diff'
This fixes on_file_diff to behave similar to
PlaybookRunnerCallbacks.on_file_diff().

    $ ansible all -m ping -i 'localhost,' -c local -C
    localhost | FAILED => Traceback (most recent call last):
      File "/home/stephenf/Source/ansible/lib/ansible/runner/__init__.py", line 297, in _executor
        exec_rc = self._executor_internal(host)
      File "/home/stephenf/Source/ansible/lib/ansible/runner/__init__.py", line 354, in _executor_internal
        return self._executor_internal_inner(host, self.module_name, self.module_args, inject, port)
      File "/home/stephenf/Source/ansible/lib/ansible/runner/__init__.py", line 509, in _executor_internal_inner
        self.callbacks.on_file_diff(conn.host, result.diff)
      File "/home/stephenf/Source/ansible/lib/ansible/callbacks.py", line 288, in on_file_diff
        if self.options.diff:
    AttributeError: Values instance has no attribute 'diff'

    $ git co fix-ansible-cli-check-mode 
    Switched to branch 'fix-ansible-cli-check-mode'
    $ ansible all -m ping -i 'localhost,' -c local -C

    localhost | success >> {
        "changed": false, 
        "ping": "pong"
    }