context.py: various fixes

- use a normal changeset instead of a context
- define _filenode instead of _node to match the else clause

mercurial/context.py

@@ -79,10 +79,10 b' class filectx(object):'
 
         if self._id:
             # if given a changeset id, go ahead and look up the file
-            self._changeset = changectx(self._repo, self._id)
+            self._changeset = self._repo.changelog.read(self._id)
             node, flag = self._repo.manifest.find(self._changeset[0], path)
-            self._node = node
             self._filelog = self._repo.file(self._path)
+            self._filenode = node
         elif self._fileid:
             # else be lazy
             self._filelog = self._repo.file(self._path)