Added auto scrolling.
Current difference is now always in viewport.

Added functions get_changes() and scroll_to_change(change) around line 767. Both new review.
We shouldn't need to rerun the full diff but I couldn't find how to retrieve the diff results.
Scrolling to a given change is quite tricky (musn't break the canvas graph, must have both pane nice).
Played with ajax.html.
It's probably no longer simple enough as an example. On the other hand it has a nice drag and drop feature. I couldn't make the "ignorews" checkbos work.

I had to update codemirror.css to have the texteditor height match the container. I tested using Opera browser and there seems to be no autosizing.