fix heads for rev 0

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Hash: SHA1

fix heads for rev 0

manifest hash: 7b2f4607f6c54b660d63ac35ba953576464e1511
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mercurial/revlog.py

@@ -159,7 +159,7 b' class revlog:'
     def heads(self):
         p = {}
         h = []
-        for r in range(self.count() - 1, 0, -1):
+        for r in range(self.count() - 1, -1, -1):
             n = self.node(r)
             if n not in p:
                 h.append(n)