Gradle reports NO-SOURCE instead of UP-TO-DATE

[sherter]

Since v3.4 Gradle reports NO-SOURCE instead of UP-TO-DATE when everything is already formatted properly. This is due to our own caching mechanism, which injects the files that need to be formatted into the task right before the task is executed (the gradle caching mechanism doesn't allow us to tell that a file is up-to-date if it was changed in the first run). Functionality is not influenced by this in any way, but users might be irritated by this message. Let's see if we can somehow trick Gradle into reporting UP-TO-DATE instead...