Investigate `mst_test_suite` failures

[snarfed]

@DavidBuchanan314 graciously integrated his exhaustive https://github.com/DavidBuchanan314/mst-test-suite into arroba via https://github.com/snarfed/arroba/blob/main/arroba/tests/mst_test_suite.py . Awesome! It has a bunch of failures right now, we should investigate.

[snarfed]

I suspect most (if not all) of these failures are bugs in our Diff class, not our MST itself, since we've been using our MST in prod happily for years, but Diff not at all.