clfilter: use empty frozenset intead of empty tuple

This will allows set operation needed for cache collaboration.

mercurial/changelog.py

@@ -124,7 +124,7 b' class changelog(revlog.revlog):'
         self._realopener = opener
         self._delayed = False
         self._divert = False
-        self.filteredrevs = ()
+        self.filteredrevs = frozenset()
 
     def tip(self):
         """filtered version of revlog.tip"""

mercurial/repoview.py

@@ -17,7 +17,7 b' def computeunserved(repo):'
     # fast path in simple case to avoid impact of non optimised code
     if phases.hassecret(repo) or repo.obsstore:
         return frozenset(repo.revs('hidden() + secret()'))
-    return ()
+    return frozenset()
 
 # function to compute filtered set
 filtertable = {'unserved': computeunserved}