Extend CI to test each commit separately in a PR

[dmnks]

Bisecting is so much easier when each commit is a self-contained change that compiles fine. Bonus points for it passing the test-suite also.

I haven't thought about it too much yet but it seems like make ci would be the best place to hook this up since it requires multiple git checkouts followed by clean builds. You don't want it to mess with your local source & build directories.