tests: avoid referring to commits by hash in a few places This patch means that the next patch can be written without manual editing of the test cases.

tests: add more verification of output to divergence tests It's not obvious what the obsmarkers should look like in these tests and adding them highlights a bug that happens when commit we create to resolve the divergence doesn't get created because it would be empty. I also added a few graphlog calls to check that the divergence-resolution commit is in the right place (has right parentage).

tests: add debugobsolete calls before many obslog calls As requested by Pierre-Yves. The main reason is to catch superfluous creation of obsmarkers, as well as any future changes to how the obsmarkers get created.