bug#64649: 30.0.50; Customize: Comments on face customization not being saved

[Mauro Aranda]

This cannot be reproduced with emacs -Q, since it involves saving the
customizations to the custom-file.

So, backup your custom-file, follow these steps with a test user, or
something.  Start Emacs and evaluate the following:

(defface foo '((t nil))
  "...")

Then: M-x customize-face RET foo
Edit at least an attribute, like the foreground color, for example.
Action the State button and "Add Comment"
Add a comment in the comment widget, like "Test"
Action the State button and "Save for Future Sessions"
Now visit the custom file: C-x C-f CUSTOM-FILE
Search for the foo customization:  '(foo ((t (:foreground "gray"))))
The "Test" comment isn't saved.

This used to work in Emacs 27, but stopped working in Emacs 28.

I think the following commit introduced this regression:

commit a4c07bc8c8201a620c4365c1d0d2cb814cc677a9
Author: Alan Mackenzie <acm@muc.de>
Date:   Fri May 1 12:23:15 2020 +0000

    Protect non-selected face spec components in custimize-face.  Fixes 
bug #40866

    * lisp/cus-edit.el (custom-face-save): If the current face widget 
is only
    displaying part of the face spec, temporarily set it to "display" 
the whole
    spec around the call to custom-face-mark-to-save.



In GNU Emacs 30.0.50 (build 27, x86_64-pc-linux-gnu, GTK+ Version
 3.24.33, cairo version 1.16.0) of 2023-07-14 built on tbb-desktop
Repository revision: e27cb447a8ceeee02428840baa29d122d2b6bd56
Repository branch: bug64347-eli
Windowing system distributor 'The X.Org Foundation', version 11.0.12201001
System Description: Ubuntu 22.04.2 LTS

Configured using:
 'configure --with-mailutils'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
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  value of $LC_NUMERIC: es_AR.UTF-8
  value of $LC_TIME: es_AR.UTF-8
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  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  show-paren-mode: t
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  mouse-wheel-mode: t
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  file-name-shadow-mode: t
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  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
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Load-path shadows:
~/code/emacs/hiddenquote/hiddenquote/hiddenquote hides 
/home/tbb/.emacs.d/elpa/hiddenquote-20210205.1401/hiddenquote
/home/tbb/.emacs.d/elpa/transient-20221019.941/transient hides 
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Memory information:
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