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…)