Introduce a diff option to the overwrite prompt

When prompted to make a decision about overwriting a file, offer the option to show a diff:

![](https://api.monosnap.com/image/download?id=44dlFK51Rk2NiPRNz8MsbiocVgdyh7)

A screencast: http://screencast.com/t/2XdeVi5YQr3