bug#21262: 25.0.50; Diff-mode gets confused about its narrowing if hunk source not found

[npostavs]

charles@aurox.ch (Charles A. Roelli) writes:

> This still affects 26.0.50.
>
> The attached change fixes it -- but I know nothing of diff-mode, so
> better ideas are welcome.  Note that the buffer will be widened while
> Emacs asks where to look for the sources, but it will return to the
> original narrowing after the user has selected a file or hit C-g.

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.