bug#66212: 30.0.50; C-x v v and vc-diff with fossil-scm

["Alfred M. Szmidt" <ams@gnu.org>]

When trying to use C-x v v in a vc-diff buffer, and when the version
control system is Fossil (this is using GNU patch):

Checking in foo...
user-error: Patch failed: can't find file to patch at input line 5
Perhaps you used the wrong -p or --strip option?
The text leading up to this was:
--------------------------
|Index: foo
|==================================================================
|--- foo
|+++ foo
--------------------------
File to patch: 
Skip this patch? [y] 
Skipping patch.
1 out of 1 hunk ignored


To reproduce, make a empty Fossil repostiroy, add and commit a file,
add a line, C-x v =, C-x v v.  vc-fossil has to be installed.


In GNU Emacs 30.0.50 (build 3, x86_64-unknown-openbsd7.3, GTK+ Version
 3.24.37, cairo version 1.17.8) of 2023-09-25 built on
 nitrogenium.mendeleev
Repository revision: df5a9a78b51f2f42d2dbb010e811a239fc014732
Repository branch: master
System Description: OpenBSD nitrogenium.mendeleev 7.3 GENERIC.MP#3 amd64

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GNUTLS GSETTINGS HARFBUZZ JPEG JSON LCMS2
LIBOTF LIBXML2 M17N_FLT MODULES NOTIFY KQUEUE PDUMPER PNG RSVG THREADS
TIFF TOOLKIT_SCROLL_BARS WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
  locale-coding-system: nil

Major mode: Fundamental

Minor modes in effect:
  global-company-mode: t
  company-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
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  minibuffer-regexp-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
~/vc-backup hides /home/ams/.emacs.d/elpa/vc-backup-1.1.0/vc-backup
~/loaddefs hides /home/ams/emacs/lisp/loaddefs

Features:
(shadow emacsbug message yank-media puny dired dired-loaddefs rfc822
mml mml-sec epa epg rfc6068 epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader vc-fossil
vc vc-dispatcher term/screen term/xterm xterm rcirc parse-time iso8601
time-date mail-queue sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils slime-asdf grep slime-quicklisp slime-fancy
slime-indentation slime-cl-indent cl-indent slime-trace-dialog
slime-fontifying-fu slime-package-fu slime-references
slime-compiler-notes-tree slime-scratch slime-presentations advice
bridge slime-macrostep macrostep slime-mdot-fu slime-enclosing-context
slime-fuzzy slime-fancy-trace slime-fancy-inspector slime-c-p-c
slime-editing-commands slime-autodoc slime-repl elp slime-parse slime
derived cl-extra help-mode easy-mmode gud apropos compile
text-property-search arc-mode archive-mode noutline outline icons pp
comint ansi-osc ansi-color hyperspec thingatpt slime-autoloads
company-oddmuse company-keywords company-etags etags fileloop
generator xref project ring company-gtags company-dabbrev-code
company-dabbrev company-files company-capf company-cmake company-xcode
company-clang company-semantic company-eclim company-template
company-bbdb company edmacro kmacro pcase cal-menu calendar
cal-loaddefs autoload loaddefs-gen radix-tree lisp-mnt finder-inf
disp-table vc-backup-autoloads package browse-url url url-proxy
url-privacy url-expand url-methods url-history url-cookie
generate-lisp-file url-domsuf url-util mailcap url-handlers url-parse
auth-source cl-seq eieio eieio-core cl-macs password-cache json subr-x
map byte-opt gv bytecomp byte-compile url-vars cl-loaddefs cl-lib rmc
iso-transl tooltip cconv eldoc paren electric uniquify ediff-hook
vc-hooks lisp-float-type elisp-mode mwheel term/x-win x-win
term/common-win x-dnd touch-screen 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 nadvice seq simple
cl-generic indonesian philippine 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 abbrev obarray oclosure cl-preloaded button
loaddefs theme-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 kqueue lcms2 dynamic-setting system-font-setting
font-render-setting cairo gtk x-toolkit xinput2 x multi-tty
move-toolbar make-network-process emacs)

Memory information:
((conses 16 214273 12959) (symbols 48 16544 0) (strings 32 53834 633)
 (string-bytes 1 1487984) (vectors 16 21835)
 (vector-slots 8 257553 8908) (floats 8 63 323) (intervals 56 279 0)
 (buffers 984 11))