The docstring of expand-file-name suggest
(directory-file-name (file-name-directory dirname))
to traverse a directory

This looks wrong to me, e.g.
(list current-directory
      (directory-file-name (file-name-directory current-directory)))
 => ("/home/youngfrog/" "/home/youngfrog")

I think it should be (file-name-directory (directory-file-name dirname))
instead. Then the example becomes:

(list current-directory
      (file-name-directory (directory-file-name current-directory)))
 => ("/home/youngfrog/" "/home/")

Am I overlooking something ?
Is this patch ok to commit ?

In GNU Emacs 25.1.50.1 (x86_64-pc-linux-gnu)
 of 2016-11-15 built on phie-fixe
Repository revision: f994c2046588b168c1a4a900879cdffaf9d02f01
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:	Ubuntu 16.04.2 LTS