phases: make _filterunknown a member function of phasecache We'd like the ability to call filterunknown on an existing phasecache instance after nodes are destroyed.

localrepo: drop `_cacheabletip` method It iss dead code now.

branchmap: drop `_cacheabletip` usage in `updatecache` Nobody overwrite the `_cacheabletip` any more. We always update the cache for the whole repo and write it to disk (or at list try to). The `updatecache` code is simplied to remove the double phase logic associated with _cacheabletip.

mq: drop `_cacheabletip` usage Strip have dedicated work around to solve the same problem, strip is even a fraction faster without that thanks to simpler update process of the branchcache.

bundlerepo: drop use of `_cacheabletip` Now that bundlerepo use a read only VFS, we do not worry about what part of the branchmap is written back to disk. Nothing is written at all.

bundlerepo: enforce reading from core repo only We do not want anything computed with the bundle overlay to be written back in the repo. Such write will likely contains invalid data. The short terms goal of this change is to drop use of `_cacheabletip` in bundle repo.

branchmap: ignore Abort error while writing cache Read only vfs can now raise Abort exception. Note that encoding.local are also a possible raiser.

vfs: add a read only vfs This read only wrapper is intended to be used bundle repo. See follow up commit for details.

branchmap: read return None in case of failure This makes a clear distinction between having read a valid cache on disk or not. This will help caches of various filtering level to collaborate.

clfilter: fallback to unfiltered version when linkrev point to filtered history On `filectx`, linkrev may point to any revision in the repository. When the repository is filtered this may lead to `filectx` trying to build `changectx` for filtered revision. In such case we fallback to creating `changectx` on the unfiltered version of the reposition. This fallback should not be an issue because `changectx` from `filectx` are not used in complex operation that care about filtering. It is complicated to work around the issue in a clearer way as code raising such `filectx` rarely have access to the repository directly. Linkrevs create a lot of issue with filtering. It is stored in revlog entry at creation time and never changed. Nothing prevent the changeset revision pointed to become filtered. Several bogus behavior emerge from such situation. Those bugs are complex to solve and not part of the current effort to install filtering. This changeset is simple hack that prevent plain crash in favor on minor misbehavior without visible effect. This "hack" is longly documented in to code itself to help people that would look at it in the future.