bug#26871: 25.2; Support revert-buffer for Customize buffers

[Phil Sainty <psainty@orcon.net.nz>]

The only way I'm aware of to refresh a Customize buffer when things
have changed externally is to kill the buffer and recreate it.

It would be convenient if `revert-buffer' was supported, and bound
to "g" as usual.




In GNU Emacs 25.2.1 (x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2017-04-22 built on shodan
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:	Ubuntu 16.04.2 LTS

Configured using:
 'configure --prefix=/home/phil/emacs/25.2/usr/local
 --with-x-toolkit=lucid --without-sound'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK DBUS GSETTINGS NOTIFY
GNUTLS LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11

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

Major mode: Dired by name

Minor modes in effect:
  tooltip-mode: t
  global-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
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Quit
user-error: Beginning of history; no preceding item
user-error: End of history; no default available

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message format-spec rfc822 mml mml-sec
password-cache epg epg-config gnus-util mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util help-fns help-mode easymenu cl-loaddefs pcase
cl-lib mail-prsvr mail-utils dired time-date mule-util tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core 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 charscript case-table epa-hook jka-cmpr-hook help
simple abbrev 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
dbusbind inotify dynamic-setting system-font-setting font-render-setting
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 90861 10521)
 (symbols 48 19761 0)
 (miscs 40 59 185)
 (strings 32 15178 4135)
 (string-bytes 1 428828)
 (vectors 16 12549)
 (vector-slots 8 437620 4562)
 (floats 8 173 65)
 (intervals 56 284 0)
 (buffers 976 19)
 (heap 1024 33986 939))