Fix line diff side-by-side line highlighting

## What does this MR do?

Fixes line highlighting in diff side-by-side view from working correctly.

The JS was looking for a class, but the class doesn't exist & instead it was a data attribute.

## Screenshots (if relevant)

![Screen_Shot_2016-09-13_at_10.26.15](/uploads/7aca730efb14d7bd6bad2a8712fe39d2/Screen_Shot_2016-09-13_at_10.26.15.png)

See merge request !6316