vc-find-root and nonexistent drives

[Juanma Barranquero]

On Windows, trying to visit a file in a non-existent drive loops in
`vc-find-root'.

The reason is that currently, `vc-find-root' does:

  (while (not (file-directory-p file))
    (setq file (file-name-directory (directory-file-name file))))

which assumes that the output of `file-name-directory' will be
different in each iteration of the while loop. That is not so when the
drive does not exist, for example:

  (file-name-directory (directory-file-name "g:/")) => "g:/"

This is not an obscure bug. "C-x C-f g:/myfile", o "emacsclient
g:/myfile" will trigger it if g: does not exist. It happens in
EMACS_22_BASE too.

             Juanma