hidden: drop cache on hiddenrevs property The `filteredrevs` function already have a cache mechanism. And this cache in invalidated at the same time than the current property cache. So we drop the cache on the property. The property itself is going to be dropped soon.

hidden: move computation in filter function There is not good reason for this computation to be handle in a different way from the other. We are moving the computation of hidden revs in the filter function. In later changesets, code that access to `repo.hiddenrevs` will be updated and the property dropped.

branchcache: add note about cache invalidation to test-keyword.t [Should've been included in aff706b3a21c.] --Kevin Bullock <kbullock@ringworld.org>