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.