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
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).