Fix *vc-diff* buffer's behavior for empty diffs.

* vc/vc.el (vc-diff-internal): Set up Diff mode even if there are
no changes to show.

Fixes: debbugs:12586

lisp/ChangeLog

 2012-10-20  Chong Yidong  <cyd@gnu.org>
 
+	* vc/vc.el (vc-diff-internal): Set up Diff mode even if there are
+	no changes to show (Bug#12586).
+
 	* eshell/esh-cmd.el (eshell-rewrite-for-command): Copy the body
 	list explicitly (Bug#12571).
 

lisp/vc/vc.el

@@ -1584,12 +1584,6 @@ Return t if the buffer had changes, nil otherwise."
     (let ((vc-disable-async-diff (not async)))
       (vc-call-backend (car vc-fileset) 'diff files rev1 rev2 buffer))
     (set-buffer buffer)
-    (if (and (zerop (buffer-size))
-             (not (get-buffer-process (current-buffer))))
-        ;; Treat this case specially so as not to pop the buffer.
-        (progn
-          (message "%s" (cdr messages))
-          nil)
     (diff-mode)
     (set (make-local-variable 'diff-vc-backend) (car vc-fileset))
     (set (make-local-variable 'revert-buffer-function)
@@ -1599,6 +1593,12 @@ Return t if the buffer had changes, nil otherwise."
     ;; bindings are nicer for read only buffers. pcl-cvs does the
     ;; same thing.
     (setq buffer-read-only t)
+    (if (and (zerop (buffer-size))
+             (not (get-buffer-process (current-buffer))))
+        ;; Treat this case specially so as not to pop the buffer.
+        (progn
+          (message "%s" (cdr messages))
+          nil)
       ;; Display the buffer, but at the end because it can change point.
       (pop-to-buffer (current-buffer))
       ;; The diff process may finish early, so call `vc-diff-finish'