clfilter: add actual repo filtering mechanism We add a `filtered` method on repo. This method return an instance of `repoview` that behaves exactly as the original repository but with a filtered changelog attribute. Filters are identified by a "name". Planned filter are `unserved`, `hidden` and `mutable`. Filtering the repository in place what out of question as it wont not allows multiple thread to share the same repo. It would makes control of the filtering scope harder too. See the `repoview` docstring for details. A mechanism to compute filtered revision is also installed. Some caches will be installed in later commit.