bug#6870: Make vc-root-diff work in more modes

[rogers-emacs]

   It makes sense to me that vc-root-diff uses "(vc-responsible-backend
default-directory)" when in a dired-mode buffer.  But since vc-root-diff
is not at all about the current buffer, why not also make that the
default for shell-mode, diff-mode, or any of the other modes for
non-version-controlled buffers that may be generated in a working copy?

                                        -- Bob Rogers
                                           http://www.rgrjr.com/

------------------------------------------------------------------------
diff --git a/lisp/vc.el b/lisp/vc.el
index 1e52a3c..abba202 100644
--- a/lisp/vc.el
+++ b/lisp/vc.el
@@ -1655,8 +1655,8 @@ saving the buffer."
     (when buffer-file-name (vc-buffer-sync not-urgent))
     (let ((backend
           (cond ((derived-mode-p 'vc-dir-mode)  vc-dir-backend)
-                ((derived-mode-p 'dired-mode) (vc-responsible-backend 
default-directory))
-                (vc-mode (vc-backend buffer-file-name))))
+                (vc-mode (vc-backend buffer-file-name))
+                (t (vc-responsible-backend default-directory))))
          rootdir working-revision)
       (unless backend
        (error "Buffer is not version controlled"))