changelog: use 'tiprev()' in 'tip()'

Since 'tip()' is just a node version of 'tiprev()' we should reuse the code.

mercurial/changelog.py

@@ -302,9 +302,7 b' class changelog(revlog.revlog):'
 
     def tip(self):
         """filtered version of revlog.tip"""
-        for i in xrange(len(self) -1, -2, -1):
-            if i not in self.filteredrevs:
-                return self.node(i)
+        return self.node(self.tiprev())
 
     def __contains__(self, rev):
         """filtered version of revlog.__contains__"""