Change 'update-file' to only write a new file if there are changes.

Hi.

Thanks for tools.namespace — really useful.

I was annoyed that `move-ns` changes the timestamps on files whose content doesn't change; this fixes that.

This is my first ever pull request (apart from an earlier one for this change that I screwed up and deleted). Apologies if anything's not right.

I couldn't find any tests to modify. If I've missed something let me know.

Regards,
Simon Katz