Performance conscious git diff load

On large files, git diff causes a big performance problem.  

This change will first load 50 lines of changes, includes unchanged lines surrounding changed lines to give context, and provide a button to load full diff reminding them how many more lines will be loaded.

![screen shot 2014-04-27 at 19 43 16](https://cloud.githubusercontent.com/assets/5281068/2812770/21ee6018-ce6e-11e3-8ba6-8073a5360f9d.png)