revlog: restore the base method

revlog: restore the base method

--- a/mercurial/revlog.py	Sat May 14 22:07:58 2011 -0700
+++ b/mercurial/revlog.py	Sun May 15 11:50:15 2011 -0500
@@ -319,6 +319,8 @@
         return self.start(rev) + self.length(rev)
     def length(self, rev):
         return self.index[rev][1]
+    def base(self, rev):
+        return self.index[rev][3]
     def chainbase(self, rev):
         index = self.index
         base = index[rev][3]