evolve: add pre-check logic for content-divergence in rewriteutil.precheck() For now, pre-check will abort if rewriting a rev create divergence (and config experimental.evolution.allowdivergence is not set to True). But this behaviour can be improved where instead of abort maybe we can confirm the user to either proceed with divergence or some other options depends on what command user is running. Changes in test file are used to overrides the pre-check for testing purpose (using experimental.evolution.allowdivergence=yes).

topic: make in-memory rebase preserve topic The override code thought that `__init__` would return the runtime instance, but it's actually the first argument to the function (the `self` argument), so the code had no effect at all before this patch. I think the bug only affected in-memory rebase because the in-working-copy rebase used the current topic, which was set correctly since 5156a67f66a6 (topics: update current topic to the topic of newly rebased commit (issue5551), 2017-06-29).