bug#25983: 25.2; set dired-directory in vc-dir mode

[Tom Tromey]

>>>>> "GM" == Glenn Morris <rgm@gnu.org> writes:

GM> Is dired-directory not basically an internal dired variable that should
GM> only be set in Dired mode, and modes derived from it, which vc-dir is not?

Various bits of code (and some other code I have seen, not part of
Emacs) seem to examine it.  And vc-dir seems dired-like.

The basic problem is that I don't think there's a good way to indicate:
"this buffer is associated with this directory, but isn't visiting a
file".  dired-directory is the only one I know of.

default-directory seems almost right but it's a bit random for some
buffers; like *shell* or *compilation*.

There's list-buffer-directory but it is just a random string, not
actually a directory name.

The backstory for this bug is that I have this handy bit of code I got
from someone on #emacs:

    ;; From fledermaus
    (defun kill-buffers-under (under)
      (interactive "DKill buffers under: ")
      (setq under (concat "^" (expand-file-name under)))
      (mapc
       (lambda (buf)
         (let ((path
                (or (buffer-file-name buf)
                    (progn
                      (set-buffer buf)
                      (and dired-directory
                           (expand-file-name dired-directory)) )) ))
           ;;(message "checking %S: %S vs |%s|" buf path under)
           (and path (string-match under path)
                (progn
                  (message "Killing buffer %s" (buffer-name buf))
                  (kill-buffer buf))) ))
       (buffer-list)))

... and I'd rather not add a special hack just for vc-dir, and then for
whatever other modes crop up.

I guess I'm being a bit lazy trying to avoid introducing a new variable.
But maybe I should?

Tom