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).
6 jobs for topic/default/in-memory-rebase in 4 minutes and 22 seconds (queued for 1 second)