> From: npostavs@users.sourceforge.net
> Cc: Dima Kogan <dima@secretsauce.net>,  21262@debbugs.gnu.org
> Date: Mon, 21 Aug 2017 20:18:39 -0400
>
> Looks like a reasonable fix to me (though I know only a little bit of
> diff-mode).  Here it is again ignoring whitespace, which I think makes
> it easier to see what is happening:
> 
> modified   lisp/vc/diff-mode.el
> @@ -874,6 +874,8 @@ diff-find-file-name
>      ;; Flush diff-remembered-files-alist if the default-directory is changed.
>      (set (make-local-variable 'diff-remembered-defdir) default-directory)
>      (set (make-local-variable 'diff-remembered-files-alist) nil))
> +  (save-restriction
> +    (widen)
>      (save-excursion
>        (unless (looking-at diff-file-header-re)
>          (or (ignore-errors (diff-beginning-of-file))
> @@ -919,7 +921,7 @@ diff-find-file-name
>  				   (file-name-nondirectory file)))
>               (set (make-local-variable 'diff-remembered-files-alist)
>                    (cons (cons fs file) diff-remembered-files-alist))
> -           file))))))
> +             file)))))))
> 
> Perhaps we should swap the save-restriction and save-excursion around
> though?  The elisp manual says:
> 
>      If you use both `save-restriction' and `save-excursion' together,
>      `save-excursion' should come first (on the outside).  Otherwise,
>      the old point value would be restored with temporary narrowing
>      still in effect.

Good point, thank you.  I'll apply the attached patch in a few days if
there's no further discussion.