bug#54034: 29.0.50; Diff prettify broken for empty files

[Matthias Meulien <orontee@gmail.com>]

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When `diff-font-lock-prettify' is non-nil, the fontification of a diff
where an empty file is added to a Git project is broken:


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where I called `vc-root-diff' from the root of an clone of emacs Git
repository after I ran the following commands:

    matthias@carbon:~/Sources/emacs$ git status
    Sur la branche master
    Votre branche est à jour avec 'origin/master'.
    
    rien à valider, la copie de travail est propre
    matthias@carbon:~/Sources/emacs$ touch empty.py
    matthias@carbon:~/Sources/emacs$ git add empty.py

As one can see from the "git diff" output, there's no line with "---"
nor "+++" signs as expected in the implementation of
`diff--font-lock-prettify':

    matthias@carbon:~/Sources/emacs$ git diff --cached
    diff --git a/empty.py b/empty.py
    new file mode 100644
    index 0000000000..e69de29bb2

Note that empty "__init__.py" files are quite common in Python based
projects.



In GNU Emacs 29.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2022-02-15 built on carbon
Repository revision: c4ca19e7eb6ed25b1f38f614ef0c2bbb7df12bc3
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 LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3
THREADS TIFF TOOLKIT_SCROLL_BARS WEBP 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: Summary

Minor modes in effect:
  highlight-changes-visible-mode: t
  shell-dirtrack-mode: t
  minions-mode: t
  global-company-mode: t
  company-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
  show-paren-mode: t
  electric-layout-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tab-bar-mode: t
  file-name-shadow-mode: t
  context-menu-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
  buffer-read-only: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
/home/matthias/.config/emacs/elpa/transient-20220130.1941/transient hides /usr/local/share/emacs/29.0.50/lisp/transient
/home/matthias/.config/emacs/elpa/dictionary-20201001.1727/dictionary hides /usr/local/share/emacs/29.0.50/lisp/net/dictionary

Features:
(shadow emacsbug sendmail sort smiley gnus-cite mm-archive mail-extr
textsec uni-scripts idna-mapping ucs-normalize uni-confusable
textsec-check gnus-bcklg gnus-async 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 gnus-cache
image-dired woman man dabbrev sh-script executable mhtml-mode css-mode
hideshow cap-words superword subword js generic meson-mode smie misearch
multi-isearch conf-mode yaml-mode make-mode rng-xsd xsd-regexp rng-cmpct
rng-nxml rng-valid nxml-mode nxml-outln nxml-rap sgml-mode facemenu rst
smerge-mode diff whitespace hl-line add-log log-view pcvs-util follow
mule-util bug-reference flyspell ox-odt rng-loc rng-uri rng-parse
rng-match rng-dt rng-util rng-pttrn nxml-parse nxml-ns nxml-enc xmltok
nxml-util ox-latex ox-icalendar org-agenda ox-html table ox-ascii
ox-publish ox goto-addr org-element avl-tree ol-eww eww xdg url-queue
mm-url ol-rmail ol-mhe ol-irc ol-info ol-gnus nnselect gnus-art mm-uu
mml2015 mm-view mml-smime smime dig gnus-sum shr pixel-fill kinsoku svg
dom ol-docview doc-view jka-compr image-mode exif ol-bibtex ol-bbdb
ol-w3m ol-doi org-link-doi dired-aux display-line-numbers hilit-chg
vc-mtn vc-hg vc-bzr vc-src vc-sccs vc-svn vc-cvs vc-rcs python vc-dir vc
bash-completion shell eglot array jsonrpc ert ewoc debug backtrace
flymake-proc flymake compile imenu company-oddmuse company-keywords
company-etags etags fileloop generator xref project company-gtags
company-dabbrev-code company-dabbrev company-files company-clang
company-capf company-cmake company-semantic company-template
company-bbdb avoid minions company pcase carbon-custom cus-edit cus-load
gnus-demon nntp gnus-group gnus-undo gnus-start gnus-dbus dbus xml
gnus-cloud nnimap nnmail mail-source utf7 netrc parse-time gnus-spec
gnus-win nnoo gnus-int gnus-range message yank-media rmc puny rfc822 mml
mml-sec epa derived epg rfc6068 epg-config mm-decode mm-bodies mm-encode
mail-parse rfc2231 rfc2047 rfc2045 ietf-drums mailabbrev gmm-utils
mailheader gnus nnheader gnus-util mail-utils range mm-util mail-prsvr
wid-edit gnus-dired dired-x dired dired-loaddefs org-capture org-refile
org ob ob-tangle ob-ref ob-lob ob-table ob-exp org-macro org-footnote
org-src ob-comint org-pcomplete pcomplete comint ansi-color ring
org-list org-faces org-entities org-version ob-emacs-lisp ob-core
ob-eval org-table oc-basic bibtex iso8601 time-date ol org-keys oc
org-compat org-macs org-loaddefs format-spec find-func cal-menu calendar
cal-loaddefs dictionary link connection advice markdown-mode
edit-indirect color thingatpt noutline outline skeleton find-file vc-git
diff-mode easy-mmode vc-dispatcher ispell comp comp-cstr warnings rx
cl-extra help-mode desktop frameset server bookmark text-property-search
pp saveplace elec-pair icomplete so-long autorevert filenotify
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 info package browse-url url url-proxy url-privacy
url-expand url-methods url-history url-cookie url-domsuf url-util
mailcap url-handlers url-parse auth-source cl-seq eieio eieio-core
cl-macs eieio-loaddefs password-cache json map url-vars seq gv subr-x
byte-opt bytecomp byte-compile cconv cl-loaddefs cl-lib iso-transl
tooltip eldoc paren electric uniquify ediff-hook vc-hooks
lisp-float-type elisp-mode mwheel term/x-win x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode lisp-mode prog-mode register page tab-bar menu-bar
rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
font-lock syntax font-core term/tty-colors frame minibuffer 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 emoji-zwj 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 sha1 md5 base64 format env code-pages
mule custom widget keymap 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 990058 38601)
 (symbols 48 42390 5)
 (strings 32 194841 12704)
 (string-bytes 1 6139210)
 (vectors 16 98218)
 (vector-slots 8 1847045 66873)
 (floats 8 689 257)
 (intervals 56 17287 491)
 (buffers 992 227))

-- 
Matthias