>> vc-find-revision-no-save also calls set-auto-mode.  So, I don't know if
>> the following patch is correct, but together with your patch it does
>> solve the original issue.
>>
>> diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
>> index f64b6c0631..c50ba132e7 100644
>> --- a/lisp/vc/vc.el
>> +++ b/lisp/vc/vc.el
>> @@ -2098,7 +2098,7 @@ vc-find-revision-no-save
>>                      ;; For non-interactive, skip any questions
>>                      (let ((enable-local-variables :safe) ;; to find `mode:'
>>                            (buffer-file-name file))
>> -                      (ignore-errors (set-auto-mode)))
>> +                      (ignore-errors (delay-mode-hooks (set-auto-mode))))
>>                    (normal-mode))
>>  	        (set-buffer-modified-p nil)
>>                  (setq buffer-read-only t))
>
> How 'bout we first consolidate the two cases into one by introducing
> a new function `set-auto-mode-for-filename` that both
> vc-find-revision-no-save and diff-mode can use?

Actually vc-find-revision-no-save when called as a command (e.g. by the
key 'f' from log-view) still should run hooks because the buffer it creates
is not internal.

Only its call from diff-syntax-fontify-hunk should not run hooks: