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.