changelog: introduce a 'tiprev' method

Accessing tiprev is a common need through the code base. It is usually done
using "len(changelog) -1". That form is tedious and error-prone. For example,
it will give wrong results on filtered changelog (if the unfiltered tip is
filtered).

As a result, we introduce a simple 'tiprev()' method to provide this exact
information in a nice way.

mercurial/changelog.py

@@ -295,6 +295,11 b' class changelog(revlog.revlog):'
         self._divert = False
         self.filteredrevs = frozenset()
 
+    def tiprev(self):
+        for i in xrange(len(self) -1, -2, -1):
+            if i not in self.filteredrevs:
+                return i
+
     def tip(self):
         """filtered version of revlog.tip"""
         for i in xrange(len(self) -1, -2, -1):