topics: add some noise to rewrittent changeset to prevent hash cycle If we have a changeset with topic `x`, we change it's topic to `y`, fine. When we change it's topic back again to `x`, we get the hash of the the obsoleted changeset which had the topic `x` initially. The same happens for few more cases like clearing the topic of a changeset which initially had no topic. This approach is influenced from cmdutil.amend and other commands (rebase, histedit, etc…)

topic-change: update the working copy along when changing topic of '.' This avoids leaving the working copy behind, removing another large parts of issue5441.

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.

topics: drop the clean argument from _changetopics() After this commit, None will represent that we want to clean the topic.