bug#64071: 28.2; smerge-diff creates *vc-diff* without setting it up

[Spencer Baugh]

1. emacs -Q
2. Create a buffer containing:
<<<<<<< left
foo
=======
bar
>>>>>>> right
3. M-x smerge-mode
4. M-x smerge-diff-upper-lower
5. The resulting *vc-diff* buffer is in diff-mode but it's not
read-only, which is unusual for a *vc-diff* buffer.  Probably there is
also other setup missing.

Relatedly, if a *vc-diff* buffer already exists, smerge-mode will use it
without updating the mode-line, so it will say, for example, "Diff from
*vc-change-log*" despite the diff being from smerge conflict resolution.

This bug is also present in Emacs 29 and trunk.

The below diff resolves the read-only part.  But maybe we want to be
creating the *vc-diff* by calling into vc, which would also fix the
mode-line part?

diff --git a/lisp/vc/smerge-mode.el b/lisp/vc/smerge-mode.el
index c39a9cc2f22..65d93c17a64 100644
--- a/lisp/vc/smerge-mode.el
+++ b/lisp/vc/smerge-mode.el
@@ -1259,6 +1259,7 @@ smerge-diff
              (if (eq status 0) (insert "No differences found.\n"))))
          (goto-char (point-min))
          (diff-mode)
+          (read-only-mode)
          (display-buffer (current-buffer) t))
       (delete-file file1)
       (delete-file file2))))


In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 
1.15.12, Xaw scroll bars)
 of 2023-02-09 built on 
Repository revision: 739b5d0e52d83ec567bd61a5a49ac0e93e0eb469
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: CentOS Linux 7 (Core)

Configured using:
 'configure --with-x-toolkit=lucid --without-gpm --without-gconf
 --without-selinux --without-imagemagick --with-modules --with-gif=no
 --with-cairo --with-rsvg'


Configured features:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON LIBXML2
MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM LUCID ZLIB

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

Major mode: Text

Minor modes in effect:
  smerge-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-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
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg rfc6068 epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
smerge-mode diff-mode easy-mmode diff 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
hashtable-print-readable backquote threads dbusbind inotify
dynamic-setting system-font-setting font-render-setting cairo x-toolkit
x multi-tty make-network-process emacs)

Memory information:
((conses 16 80621 6370)
 (symbols 48 11294 1)
 (strings 32 28957 2105)
 (string-bytes 1 956297)
 (vectors 16 13522)
 (vector-slots 8 187949 10390)
 (floats 8 30 39)
 (intervals 56 256 0)
 (buffers 992 12)
 (heap 1024 13825 2912))