Diff bot should warn if VIs are included in a pull request but no changes have been made

[rtzoeller]

It is easy to unintentionally save a VI without changes, or make a change to a VI and then undo it while re-saving. Since it can be difficult to diff VIs locally without additional setup, these changes tend to sneak into pull requests.

The diff bot (and underlying diff tools) could be modified to report back when a VI does not have any changes, and this information could be bubbled up to the pull request explicitly, instead of requiring the reviewer to check the diffs manually.