manifest: use list(dict) instead of dict.keys() to get a list of keys

Differential Revision: https://phab.mercurial-scm.org/D2317

mercurial/manifest.py

@@ -1019,7 +1019,7 b' class treemanifest(object):'
 
         # yield this dir's files and walk its submanifests
         self._load()
-        for p in sorted(self._dirs.keys() + self._files.keys()):
+        for p in sorted(list(self._dirs) + list(self._files)):
             if p in self._files:
                 fullp = self._subpath(p)
                 if match(fullp):