emacs-29 733cdeabfb: Don't use diff-mode buffer as a patch when it's visiting a file (bug#59962)

[Juri Linkov]

branch: emacs-29
commit 733cdeabfb91520584ab88253292c1451f295bae
Author: Juri Linkov <juri@linkov.net>
Commit: Juri Linkov <juri@linkov.net>

    Don't use diff-mode buffer as a patch when it's visiting a file (bug#59962)
    
    * lisp/vc/vc.el (vc-deduce-fileset-1): Don't call diff-vc-deduce-fileset
    for diff-mode when buffer-file-name is non-nil.  This is because in this 
case
    a file with a diff might be committed to VCS.  So don't use it as a patch
    to commit with 'C-x v v'.
---
 lisp/vc/vc.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 328d33040d..690c907c77 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1135,7 +1135,7 @@ BEWARE: this function may change the current buffer."
       (vc-dir-deduce-fileset state-model-only-files))
      ((derived-mode-p 'dired-mode)
       (dired-vc-deduce-fileset state-model-only-files not-state-changing))
-     ((derived-mode-p 'diff-mode)
+     ((and (derived-mode-p 'diff-mode) (not buffer-file-name))
       (diff-vc-deduce-fileset))
      ((setq backend (vc-backend buffer-file-name))
       (if state-model-only-files