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

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