manifestdict: remove unnecessary dictionary copy

No need to copy the dict, dict.__init__() will do that for us.
It was responsible for a non-negligeable waste of time during a qpush of an
-mm queue on the kernel repo.

mercurial/manifest.py

@@ -20,7 +20,7 b' class manifestdict(dict):'
     def set(self, f, flags):
         self._flags[f] = flags
     def copy(self):
-        return manifestdict(dict.copy(self), dict.copy(self._flags))
+        return manifestdict(self, dict.copy(self._flags))
 
 class manifest(revlog.revlog):
     def __init__(self, opener):