clfilter: ensure context raise RepoLookupError when the revision is filtered

Currently the code path of `changectx(filteredrepo, rev)` call
`filteredrepo.changelog.node(rev)`. When `rev` is filtered this raise an
unhandled `IndexError`. This case now raise a `RepoLookupError` as other
error case do.

mercurial/context.py

@@ -25,8 +25,12 b' class changectx(object):'
         self._repo = repo
 
         if isinstance(changeid, int):
+            try:
+                self._node = repo.changelog.node(changeid)
+            except IndexError:
+                raise error.RepoLookupError(
+                    _("unknown revision '%s'") % changeid)
             self._rev = changeid
-            self._node = repo.changelog.node(changeid)
             return
         if isinstance(changeid, long):
             changeid = str(changeid)