diff improvement

<img width="504" alt="Screenshot 2020-05-07 at 15 12 49" src="https://user-images.githubusercontent.com/776816/81298607-46b40e00-9075-11ea-862e-5abf8dbfffbf.png">

1. show filename of file diffed in title
2. dont show 'no text file' as if it was a diff for binary files 