- handle invalid UTF-8 in file diff (bsc#907679)

There is a shortcut for this, see `String#scrub` http://ruby-doc.org/core-2.1.0/String.html#method-i-scrub

Alternatively you can add `:replace` parameter and replace invalid bytes by a replacement character (like �), i.e. `:replace => "�"`. That won't silently hide the problematic characters and user will see that something is broken there.