Added a checkbox to toggle line wrapping in diff.

This is a better solution for [7488](https://github.com/gitlabhq/gitlabhq/pull/7488).

Before:
![before](https://cloud.githubusercontent.com/assets/6962409/4056773/577b8930-2dbf-11e4-8cd3-0a7aafbed72a.png)

After:
![after](https://cloud.githubusercontent.com/assets/6962409/4056775/6259df96-2dbf-11e4-914f-32a8ebc92eb4.png)

By default, the text is not wrapped, as it was previously. The result of wrapping is the same as in [7488](https://github.com/gitlabhq/gitlabhq/pull/7488).

Should taste better with [7611](https://github.com/gitlabhq/gitlabhq/pull/7611) =)