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).
6 jobs for topic/default/in-memory-rebase in 4 minutes and 22 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed doc #52955

00:00:25

passed flake8 #52950

00:00:22

passed tests-py2-cext #52951

00:04:19

passed tests-py2-pure #52952

00:04:22

passed tests-py3-cext #52953

00:02:57

passed tests-py3-pure #52954

00:02:25