vfs: ensure closewrapbase fh doesn't escape by entering context manager

I'm not sure if there's a problem in practice here, as there's no test failure
either way.  The __exit__() and close() methods raise an exception, so maybe
__exit__() and close() are being called directly on the underlying handle when
delayclosedfile is used on a context manager?  I doubt that was intended.

mercurial/vfs.py

@@ -518,7 +518,8 b' class closewrapbase(object):'
         return delattr(self._origfh, attr)
 
     def __enter__(self):
-        return self._origfh.__enter__()
+        self._origfh.__enter__()
+        return self
 
     def __exit__(self, exc_type, exc_value, exc_tb):
         raise NotImplementedError('attempted instantiating ' + str(type(self)))