Github added a nice feature to their STL viewer: Diff. This feature makes it possible to view the differences between revisions of an STL file. From their site:
There are two modes to figure out what you’re looking at. By default, we select “Highlight”, a mode that highlights the removed parts in red and the added parts in green, leaving what is unchanged as a wireframe. Also available is the revision slider, which lets you transition between the current version of the file, and the previous one with a small slider at the top of the viewer.