hidden: move hiddenrevs set on the repository
This set is always accessed through the repo for now. Having this set
carried by the changelog make it complicated to:
- initialize it, computing hidden set may involve revset call
- lazy compute it, (1) only the changelog can detect someone access it,
(2) only the repo have enought knowledge to compute it.
In later version I expect he changelog to apply filtering itself and the set to
be carried by changelog again.