bug#6273: dired-current-directory with `t' arg returns empty string in top-level mount-point "/"

[MON KEY]

(dired-current-directory t) returns the empty string
when in a `top-level' mount point, e.g. "/"

(progn
  (dired "/" )
  (dired-current-directory t))
 => ""

,---- (documentation 'dired-current-directory)
| {...}
| This returns a string with trailing slash, like `default-directory'.
| Optional argument means return a file name relative to `default-directory'.
`----

In the case at hand it does not return a string with trailing slash...
and appears in contrast to the return value of following forms:

(progn
  (dired "/")
  (file-relative-name default-directory))
 => "./"

(progn
  (dired "/")
  (file-relative-name (dired-default-directory)))
 => "./"

which are likewise at odds with each of the following:

(progn
  (dired "/")
  (dired-make-relative (dired-default-directory)))
  => ""

(progn
  (dired "/")
  (dired-make-relative (dired-make-absolute default-directory)))
 => "/"

So which is the correct return value? Relatively speaking...

GNU Emacs 23.2.1 (i686-pc-linux-gnu, GTK+ Version 2.20.0) of 2010-05-10

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