path compare

*Issue*
http://feedback.gitlab.com/forums/176466-general/suggestions/4300635-option-to-compare-specific-files-directories-in-th
*Depends on* 
https://github.com/gitlabhq/gitlab_git/pull/19
*Preview*
![screenshot from 2013-12-25 17 45 02](https://f.cloud.github.com/assets/1488195/1808665/afaa7c8e-6d6b-11e3-9c56-991c497b187b.png)