revlog: make chainbase cache its result for the correct revision

Previously, as 'rev' was our iterator, we were always caching the chain base
for the second revision of the chain, or for the base itself.

mercurial/revlog.py

@@ -877,10 +877,11 b' class revlog(object):'
             return base
 
         index = self.index
-        base = index[rev][3]
-        while base != rev:
-            rev = base
-            base = index[rev][3]
+        iterrev = rev
+        base = index[iterrev][3]
+        while base != iterrev:
+            iterrev = base
+            base = index[iterrev][3]
 
         self._chainbasecache[rev] = base
         return base