topic: use the 'topic' revset in namespace This revset performance just got improved.

topic-revset: update the revset to no longer build changectx Atribute access is expensive, and changectx are very expensive to build, so we skip these for better performance. Before: ! wall 0.012195 comb 0.020000 user 0.020000 sys 0.000000 (best of 217) After: ! wall 0.008816 comb 0.010000 user 0.010000 sys 0.000000 (best of 303) (Before this changeset parent) ! wall 0.213261 comb 0.210000 user 0.210000 sys 0.000000 (best of 45)

topic-revset: changectx creation in the revset The current mutability filter is very expensive, we use a more direct and efficent way to do so: Before: ! wall 0.213261 comb 0.210000 user 0.210000 sys 0.000000 (best of 45) After: ! wall 0.012195 comb 0.020000 user 0.020000 sys 0.000000 (best of 217)

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…)