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

[Alexis]

Since the documentation for `dired-current-directory' states:

"Optional argument means return a file name relative to
`default-directory'"

and there is no file name to return in the case that
`dired-current-directory' and `default-directory' are the same,
an empty string seems to be a reasonable return value here.

More generally, regarding the various values returned in each of the
examples provided, my feeling is that unless Emacs contains code in
which these various return values directly interact with each other to
produce obviously incorrect results, this bug should either be closed or
tagged 'wishlist'.