diff-mode: Optimize source language syntax highlighting

* lisp/vc/diff-mode.el (diff--get-revision-properties):
Never pass FILE to 'diff-syntax-fontify-props'; assume that
'vc-find-revision-no-save' has called the major mode function
already (bug#71604).

Author: dgutov

lisp/vc/diff-mode.el

@@ -2883,9 +2883,6 @@ and the position in MAX."
          (buffer (cdr entry)))
     (if (buffer-live-p buffer)
         (progn
-          ;; Don't re-initialize the buffer (which would throw
-          ;; away the previous fontification work).
-          (setq file nil)
           (setq diff--cached-revision-buffers
                 (cons entry
                       (delq entry diff--cached-revision-buffers))))
@@ -2905,7 +2902,8 @@ and the position in MAX."
     (diff--cache-schedule-clean)
     (and buffer
          (with-current-buffer buffer
-           (diff-syntax-fontify-props file text line-nb)))))
+           ;; Major mode is set in vc-find-revision-no-save already.
+           (diff-syntax-fontify-props nil text line-nb)))))
 
 (defun diff-syntax-fontify-hunk (beg end old)
   "Highlight source language syntax in diff hunk between BEG and END.