Visual PCB diff

This could be done in python scripting, compare current open board, to an specific pcb file (or older git revision)

I think of two ways:

1) Visual diff, layer by layer xor -> image production
2) Objet diff, track by track, module by module: we try to match every track to each other (by start/end/net) and then build a list of different tracks.
2.1) Then we must find how to visualize/report that in a way that's useful. a) Plotting everything in gray, and plotting different tracks in colors?..