Prevent type exception on concatenation if diffstat returns None. This will most often occur if diffstat is not installed in the target platform, though may also happen in other cases where diffstat fails to execute. Signed-off-by: Sean Dague <sean@dague.net>

#!/bin/sh
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a       
echo a > a 
hg commit -Ama 

hg parents -r 0 doesnotexist
true