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().
Merge pull request #2221 from sfromm/fix-ansible-cli-check-mode

Fix CliRunnerCallbacks.on_file_diff in callbacks.py