obshistory: add _originmarkers() function to be used for obslog --origin

evolve: rename `evolve_source_*` to `divergence_source_*` This makes it clearer that this comes from divergence resolution.

evolve: mark new hash-salting extras keys as uninteresting for effect flag The user doesn't care if the `evolve_source_local`/`evolve_source_other` values in the extras changed, so this patch adds them to the blacklist that already exists for that purpose. This should also fix the flakiness we've seen in some tests. The flakiness seemed to stem from `mercurial.obsutil.geteffectflag()` not sorting extras keys. That means we ran into it relatively often for the `evolve_source_local`/`evolve_source_other` keys because there were two of them. That sorting bug should be fixed upstream.

evolve: handle relocation during divergence resolution producing no changes When resolving divergence and the two divergent commits have different parents, we start by rebasing one of them to have the same parent as the other. That step can result in no changes to commit. When it does, we would crash with a TypeError before this patch. This patch fixes it by instead creating an empty commit in that scenario. The existing code then continues to attempt to merge it, which produces no changes, and no commit is created on top. The other side of the divergence is marked as successor as usual, so orphans from the side that became empty will be evolved to the right place (see test).

tests: show crash from divergence resolution resulting in empty commit When relocating a commit results in an empty commit (because the changes are already in the destination), we get a TypeError from evolve.

evolve: always create commit when resolving divergence When resolving content-divergence, the final commit we create may end up empty (which means that Mercurial won't even create it). We've had code for handling that in evolve ever since 41bf6c27a122 (evolve: stabilize now handle conflicting changeset, 2012-08-23). However, that resolved the issue by marking on the divergent commits as successor. As Pierre-Yves has pointed out (in other code reviews), we should instead be creating a new successor. So that's what this patch does. It does that by setting `ui.allowemptycommit` while creating the final commit. However, that is not enough, because we may end up creating the same nodeid as already existed (we'd then end up trying to mark the "new" commit a successor of itself). To solve that, we add some salt to the commit extras. That salt affects lots of tests.

tests: replace an X+X revset with something meaningful The change in aab2afc2bd82 seem to have been make too hastily.

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