Update rfc.sh to assert divergence.

assert_diverge didn't really _assert_ before, this change
causes it to exit if there are no differences from
origin/$branch.

Change-Id: I4b7d00425746c7f5eac4cfad20c89b75b38ac686
Reviewed-on: https://review.openstack.org/274
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins

tools/rfc.sh

@@ -91,13 +91,16 @@ rebase_changes()
 
 assert_diverge()
 {
-    git diff origin/$branch..HEAD | grep -q .;
+    if ! git diff origin/$branch..HEAD | grep -q .
+    then
+	echo "No changes between the current branch and origin/$branch."
+	exit 1
+    fi
 }
 
 
 main()
 {
-
     set_hooks_commit_msg;
 
     check_remote;