revlog: fix heads performance regression

mercurial/revlog.py

@@ -781,10 +781,10 b' class revlog(object):'
                 return [nullid]
             ishead = [1] * (count + 1)
             index = self.index
-            for r in self:
+            for r in xrange(count):
                 e = index[r]
                 ishead[e[5]] = ishead[e[6]] = 0
-            return [self.node(r) for r in self if ishead[r]]
+            return [self.node(r) for r in xrange(count) if ishead[r]]
 
         if start is None:
             start = nullid