hidden: invalidate hiddenrevs when needed The `hiddenrevs` cache is volatile too (It use content from `obscache`). When unsure it is invalidated when necessary. In a near future, the cache will probably be moved to `revsfiltercache`

cache: group obscache and revsfiltercache invalidation in a single function Both caches are very volatile and needs invalidation on the same kind of event. revsfiltercache actually depends on the content of revsfiltercache.

clfilter: use filtering in `visibleheads` This is the second real use of changelog filtering. The change is very small to allow testing the new filter with a setup close to the original one. We replace custom post processing on `heads`function by call to the standard code pass on a filtering repo. In later coming will have wider usage of filtering that will make the dedicated function useless.