master de7d9e1: Clarify that 'diff-error' is part of Emacs 28.1

[Basil L. Contovounesios]

branch: master
commit de7d9e1f88da08abf3883d585a69d52fbdd61963
Author: Protesilaos Stavrou <info@protesilaos.com>
Commit: Basil L. Contovounesios <contovob@tcd.ie>

    Clarify that 'diff-error' is part of Emacs 28.1
    
    * lisp/vc/diff-mode.el (diff-error): Add :version tag (bug#44727).
---
 lisp/vc/diff-mode.el | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 5aeb8fe..0a90613 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -403,7 +403,8 @@ well."
   '((((class color))
      :foreground "red" :background "black" :weight bold)
     (t :weight bold))
-  "`diff-mode' face for error messages from diff.")
+  "`diff-mode' face for error messages from diff."
+  :version "28.1")
 
 (defconst diff-yank-handler '(diff-yank-function))
 (defun diff-yank-function (text)