bundlerepo: use native str when peeking in __dict__

# skip-blame since it's just a string constant prefix

Differential Revision: https://phab.mercurial-scm.org/D1892

mercurial/bundlerepo.py

@@ -402,7 +402,7 b' class bundlerepository(localrepo.localre'
         # manifestlog implementation did not consume the manifests from the
         # changegroup (ex: it might be consuming trees from a separate bundle2
         # part instead). So we need to manually consume it.
-        if 'filestart' not in self.__dict__:
+        if r'filestart' not in self.__dict__:
             self._consumemanifest()
 
         return self.filestart