manifest: correct readdelta() according to parentdeltas

mercurial/manifest.py

@@ -36,6 +36,8 b' class manifest(revlog.revlog):'
 
     def readdelta(self, node):
         r = self.rev(node)
+        if self._parentdelta:
+            return self.parse(mdiff.patchtext(self.revdiff(self.deltaparent(r), r)))
         return self.parse(mdiff.patchtext(self.revdiff(r - 1, r)))
 
     def read(self, node):