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.

mercurial/context.py

@@ -418,7 +418,26 b' class filectx(object):'
 
     @propertycache
     def _changectx(self):
+        try:
         return changectx(self._repo, self._changeid)
+        except error.RepoLookupError:
+            # 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 complexe operation that care about
+            # filtering.
+            #
+            # This fallback is a cheap and dirty fix that prevent several
+            # crash. It does not ensure the behavior is correct. However the
+            # behavior was not correct before filtering either and "incorrect
+            # behavior" is seen as better as "crash"
+            #
+            # Linkrevs have several serious troubles with filtering that are
+            # complicated to solve. Proper handling of the issue here should be
+            # considered when solving linkrev issue are on the table.
+            return changectx(self._repo.unfiltered(), self._changeid)
 
     @propertycache
     def _filelog(self):