topic-change: cleanup the locking and transaction mechanism The previous code did not garanted we would release all lock (and was a bit more complicated than needed.

topics: use a dict for successors of changesets produced by topic change This patch adds a dictionary to store the successors of changesets which got obsoleted when we change it's topic. This is an improvement in the fix for issue5441.

topic: further simplify the clear logic We just set 'topic' to None and the rest should follow.