Recomputing the filtered revisions at every access to changelog is far too
expensive. This changeset introduce a cache for this information. This cache is
hold by the repository (unfiltered repository) and invalidated when necessary.
This cache is not a protected attribute (leading _) because some logic that
invalidate it is not held by the local repo itself.
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.
