Hi, I've pushed my code on github so others can use it and try it out. I'm using it constantly and it feels really stable. https://github.com/v01d/geany-livediff
The plugin shows changed/modifed/added colored markers on the margin of geany's editor windows. It currently works against the last saved version on disk of a regular file and also against files versioned in a git repository (using libgit). When a document is a regular file, the "old version" reference is reset whenever the file is reloaded (either manually or by geany). In the case of git versioned files, the diff is performed between the editor contents and the last revision of this file on the current branch. If the head reference changes (either by commiting or changing a branch), the plugin recognizes this and updates the markers accordingly (it uses a timer for this).
The markers themselves are not pretty so if anyone wants to improve that it would be great.
Matias