bug#30663: 27.0.50; DirectoryVariables not refreshing on revert-buffer in dired-mode

[Christian Johansson <christian@mediastrategi.se>]

If you create this file .dir-locals.el in a directory A:
((nil . (
          (test-variable . "My-Value 2")
          )))

And then open the directory A containing this file, when the 
confirmation of unsafe variables comes up, approve of using the variable 
value.

Now check the value of variable "test-variable" with (decribe-variable) 
inside the (dired-mode) buffer, it's value should be the expected one.

Now change the value to "My-Value 3" in the .dir-locals.el file and save 
it.

Switch to the (dired-mode) buffer again and glick g to do a 
(revert-buffer). Now inspect the same variable again, it should be 
"My-Value 3" but it is "My-Value 2"

(revert-buffer) refreshes DirectoryVariables in most modes but not in 
(dired-mode)


In GNU Emacs 27.0.50 (build 5, x86_64-apple-darwin17.4.0, NS 
appkit-1561.20 Version 10.13.3 (Build 17D102))
  of 2018-02-28 built on Christians-iMac
Repository revision: 108ce84432d597f92637ea74bd0a094224d157de
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.3

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
user-error: The mark is not set now, so there is no region
Making completion list...
delete-backward-char: Text is read-only
Making completion list... [2 times]

Configured features:
RSVG IMAGEMAGICK NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS
THREADS LCMS2

Important settings:
   value of $LANG: sv_SE.UTF-8
   locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
   tooltip-mode: t
   global-eldoc-mode: t
   eldoc-mode: t
   electric-indent-mode: t
   mouse-wheel-mode: t
   tool-bar-mode: t
   menu-bar-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   blink-cursor-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   line-number-mode: t
   transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize mule-util term/common-win
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core term/tty-colors frame cl-generic
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote kqueue cocoa ns lcms2
multi-tty make-network-process emacs)

Memory information:
((conses 16 204092 10722)
  (symbols 48 19937 1)
  (miscs 40 43 158)
  (strings 32 28820 2211)
  (string-bytes 1 760839)
  (vectors 16 35104)
  (vector-slots 8 714916 18226)
  (floats 8 48 68)
  (intervals 56 217 0)
  (buffers 992 12))