tests: update section headers and comments in obslog-related tests

tests: use common obshistory_setup.sh script instead of manual setup This is now consistent with other test-evolve-obshistory*.t tests.

tests: remove "second diverged revision" from a phase divergence test It was basically testing the same thing twice with a different comment.

obshistory: omit duplicate notes of obsolescence operations When folding changesets, it's possible to provide a --note flag to e.g. fold or prune, which will store a note for that operation. But since there would be more than one marker for the fold operation and the provided note would be stored in all of them, _markersnotes() previously would return the same text message many times. We don't really want to see the same thing over and over, so let's make notes a set(). This will also make unrelated operations with the same note show the message only once, which is probably an improvement too.

changelog: adjust The changes only apply to newly created sqlite database. There are code pathes that drop existing ones, but that's a more complicated condition.

changelog: mention the sqlite fix

evolve: extract function for relocating divergent commit There was some duplication here, so let's extract to a function. I'm going to add a little bit more shared code next.