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

[Juri Linkov <juri@linkov.net>]

> 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.
>
> @@ -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)))))

Thanks, looks correct.