Fix bug: C-x v v discarded existing log message.

* lisp/vc/vc-dispatcher.el (vc-log-edit):
Don't clobber an already-existing log message.

Fixes: debbugs:17884

lisp/ChangeLog

+2014-07-12  Paul Eggert  <eggert@cs.ucla.edu>
+
+	Fix bug: C-x v v discarded existing log message (Bug#17884).
+	* vc/vc-dispatcher.el (vc-log-edit):
+	Don't clobber an already-existing log message.
+
 2014-07-10  Glenn Morris  <rgm@gnu.org>
 
 	* vc/log-edit.el (log-edit-changelog-entries):

lisp/vc/vc-dispatcher.el

@@ -596,7 +596,7 @@ NOT-URGENT means it is ok to continue if the user says not to save."
   (setq default-directory
 	(buffer-local-value 'default-directory vc-parent-buffer))
   (log-edit 'vc-finish-logentry
-	    t
+	    (= (point-min) (point-max))
 	    `((log-edit-listfun . (lambda ()
                                     ;; FIXME: Should expand the list
                                     ;; for directories.