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

clfilter: add impactable filter The `mutable` filter still have some chance to get invalidated. This will happen when: - you garbage collect hidden changeset, - public phase is moved backward, - something is changed in the filtering (this could be fixed) So we introduce an even more stable filtering set: everything with a revision number egal or higher than the first mutable changeset is filtered. The only official use of this filter is for branchcache.

clfilter: add mutable filtering It filters all mutable changesets, leaving only public changeset unfiltered. This filtering set is expected to be much more stable that the previous one as public changeset are unlikely to disapear. The only official use of this filter is for branchcache.