bug#51016: 28.0.50; 'diff-font-lock-prettify' breaks display of outline headers

[Matthias Meulien]

When `diff-font-lock-prettify' is `t', outline headers aren't displayed
at the beginning of a line.

The following screenshot displays a
*vc-diff* buffer with outline minor mode enabled and body hidden with
and without `diff-font-lock-prettify' being `t':

Recipe to reproduce:

1. Set `diff-font-lock-prettify' to `t' and enable `outline-minor-mode'
in `diff-mode'; For example:

(add-hook 'diff-mode-hook
          (lambda ()
            (setq diff-font-lock-prettify t)
            (outline-minor-mode)))

2. In a *vc-change-log* buffer for emacs sources, find the commit with
hash a8de88e330. Press = (`log-view').

3. Press C-c @ C-t (`outline-hide-body') while in the *vc-diff*
buffer. Observe that outline headers are splitted in multiple lines.


In GNU Emacs 28.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo 
version 1.16.0)
 of 2021-09-13 built on carbon
Repository revision: 8c80430824d52e5dfae97db2669b419621817956
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --with-native-compilation'

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

Important settings:
  value of $LANG: fr_FR.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Message

Minor modes in effect:
  goto-address-mode: t
  highlight-changes-visible-mode: t
  show-paren-mode: t
  shell-dirtrack-mode: t
  minions-mode: t
  mml-mode: t
  desktop-save-mode: t
  save-place-mode: t
  electric-pair-mode: t
  icomplete-mode: t
  global-so-long-mode: t
  global-auto-revert-mode: t
  auto-insert-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-layout-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tab-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  window-divider-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  auto-fill-function: message-do-auto-fill
  transient-mark-mode: t
  abbrev-mode: t

Load-path shadows:
/home/matthias/.config/emacs/elpa/transient-20210919.1006/transient hides 
/usr/local/share/emacs/28.0.50/lisp/transient
/home/matthias/.config/emacs/elpa/flymake-1.2.1/flymake hides 
/usr/local/share/emacs/28.0.50/lisp/progmodes/flymake
/home/matthias/.config/emacs/elpa/project-0.7.1/project hides 
/usr/local/share/emacs/28.0.50/lisp/progmodes/project
/home/matthias/.config/emacs/elpa/dictionary-20201001.1727/dictionary hides 
/usr/local/share/emacs/28.0.50/lisp/net/dictionary

Features:
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debbugs soap-client url-http url-auth url-gw rng-xsd xsd-regexp
debbugs-browse dabbrev cl-print cus-start texinfo texinfo-loaddefs
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smerge-mode diff gnus-fun flow-fill mm-archive view sort smiley
gnus-cite mail-extr gnus-async gnus-bcklg qp gnus-ml disp-table
gnus-topic nndraft nnmh nnfolder utf-7 epa-file gnutls network-stream
nsm gnus-agent gnus-srvr gnus-score score-mode nnvirtual gnus-msg
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rng-match rng-dt rng-util rng-pttrn nxml-parse nxml-ns nxml-enc xmltok
nxml-util ox-latex ox-icalendar ox-html table ox-ascii ox-publish ox
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message rmc puny rfc822 mml mml-sec epa derived epg rfc6068 epg-config
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
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autoinsert cc-mode cc-fonts cc-guess cc-menus cc-cmds cc-styles cc-align
cc-engine cc-vars cc-defs generic-x face-remap proof-site
proof-autoloads comp comp-cstr warnings rx cl-extra help-mode info
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term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
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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 cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
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hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process
native-compile emacs)

Memory information:
((conses 16 1700523 255710)
 (symbols 48 38830 103)
 (strings 32 227548 67913)
 (string-bytes 1 7153152)
 (vectors 16 75900)
 (vector-slots 8 1725779 101458)
 (floats 8 2990 1100)
 (intervals 56 136558 2364)
 (buffers 992 150))

-- 
Matthias