Subject: 29.0.50; Diff prettify broken for empty files

From: Matthias Meulien <[email-masked]>
Date: Thu, 17 Feb 2022 08:47:55 +0100

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:

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:
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textsec uni-scripts idna-mapping ucs-normalize uni-confusable
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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
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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