side-by-side diff in full page width

Rescue from vertical scroll on side-by-side diff for high-resolution screen
### before
![before](https://cloud.githubusercontent.com/assets/1488195/3520689/ad480e44-0734-11e4-9e6f-39e81bafd332.png)

### after
![after](https://cloud.githubusercontent.com/assets/1488195/3520691/b2d056d2-0734-11e4-958e-31b847504105.png)