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>

Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory