bug#71604: diff--get-revision-properties calls set-auto-mode twice

[Dmitry Gutov <dmitry@gutov.dev>]

X-Debbugs-Cc: Juri Linkov <juri@linkov.net>
Tags: patch

vc-find-revision-no-save calls set-auto-mode, and then we repeat that 
call in diff-syntax-fontify-props.

Which is a problem with CC Mode because it parses the buffer inside the 
major mode function, and ends up doing that twice, at least the first 
time the buffer is initialized before being saved to revision cache. Not 
exactly double the work, but more work nevertheless.

This is a split off from https://debbugs.gnu.org/61667#587. I thought I 
recalled an existing report of vc-diff being slow, but apparently not.

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index c0269d882b9..cc5509ca347 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2883,9 +2883,6 @@ diff--get-revision-properties
           (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 @@ diff--get-revision-properties
      (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.