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).

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