revset: retrieve hidden from filteredrevs This prepare the dropping of the `repo.hiddenrevs` property

hidden: use both parents of working directory If we are merging with and extinct revision, this extinct revision should not be hidden.

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>

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.

run-tests.py: fix handling of symlink to the right python Before: a symlink for python in BINDIR was sometimes created, but it was never updated when a different Python was used and it was never removed. An invalid python could thus be left around and used when testing with --local. Now: the symlink is removed when wrong and created when necessary. The mechanism for finding the right name (python or python.exe) also had to be simplified and made more explicit.

log: use "hidden" filtering instead of manual check at display time When log is not given the --hidden option, hidden revision are not shown. We move the implementation from manual checking at display time to changelog filtering. This is the first official usage of the hidden filtering.

clfilter: introduces a hidden filter This filter exclude all hidden revision. We plan to use this filter to hide revision instead of manually checking contents of the hidden revisions set.