branching: fix wrong merge conflict resolution from 13dfad0f9f7a

13dfad0f9f7a merged stable into default, but accidentally added the
_blockingreader class from stable (but deindented) instead of merging the
changes from stable (2fe4efaa59af) into the existing _blockingreader class.
This resulted in the _blockingreader being there two times.
15 jobs for topic/default/branching in 27 minutes and 37 seconds (queued for 2 seconds)