Difftastic would not solve the issue described by madeofpalk because it still highlights the added comma. You need a diff tool that can distinguish between optional and required syntax. So far I am not aware of any tool that supports this, except the one I am working on (SemanticDiff).
Certainly the idea has been suggested many times. I think people end up formatting both before/after and doing a diff on formatted before against formatted after. I've done that.