figleaf: compute+display differential coverage

[warner]

It would be great if the buildbot produced, for each new build, a description of lines of code that gained or lost test coverage.

I just made a change to the Makefile to include the raw figleaf pickle in the directory that gets uploaded to the webserver, so from here on out we should have machine-parsable historical data. (for the builds until this point, we only have rendered HTML for each build).

I suppose the right approach is to have a 'render-figleaf-delta' target, which starts by trying to download the pickle file from the previous build, then have a script to construct the deltas, then render those deltas into more HTML, then include the delta HTML (as well as a machine-readable delta pickle) in the next batch of uploaded data.