compare-with-snapshot is a KDE service menu which adds a 'Compare with Snapshot' item to the context menu in the Dolphin file manager when right-clicking on a file. This requires you to be running the KDE desktop environment, using btrfs as your file system, and have the snapper package installed. You must also have configured snapper to manage snapshots for the btrfs subvolume containing the file that you right-clicked upon.
Selecting 'Compare with Snapshot' will pop up a window showing all the snapshots where the contents of that file actually changed. Selecting one of those snapshots will pop open the RubyMine diff utility showing the changes for the selected file between the time of the selected snapshot and its current state.
Eventually, I'll add an installation script.