WIP: Diff view

[fstachura]

Note: this is work in progress, not ready for review. Comments are welcome, but I don't expect a full review just yet. Mostly posting this to show that there is some slow progress on this feature.

Right now, the feature only supports a split diff view. This neatly solves the "which version should an identifier link to" problem.

TODO:

• Identifiers popup is not aware of the diff view at all
• Coherent interface - right now you need to first visit a file, then check "diff mode", then select a version from the sidebar
• Linking to lines doesn't work
• CSS is still slightly broken

How the diffs are generated:

• Take sources of a and b, put both of them through the tokenizer and initial filters
• Take a diff of a and b in a special format (see get_diff in script.sh) that basically only shows where to add empty lines, and which lines to mark as added/deleted
• Pass sources of both files to a subclass of Pygments's HtmlFormatter, that adds empty lines/colorizes lines based on the diff information.

There is a slight efficiency problem with this approach - files can be very similar, but both will get tokenized and formatted separately. And IIRC formatting can take a lot of time.
I don't see a good way of solving that, maybe something smart could be done with the lexer/formatter.
The only optimization that could be worth it, is caching identifiers looked up in the first file for the second file (not implemented yet, and I don't know if it would make sense). I think it would also make sense to limit the size of diffed files - no one should do diffs on the 20 megabytes amdgpu register definitions.

Closes: #25