show diff in submodules

![screenshot at 28 17-09-06](https://cloud.githubusercontent.com/assets/1488195/3720831/2367e58e-165a-11e4-9fa4-bdefaa7d00fc.png)
![screenshot at 28 17-09-37](https://cloud.githubusercontent.com/assets/1488195/3720833/2900e428-165a-11e4-9826-5687e44d8b1d.png)

depends on https://github.com/gitlabhq/gitlab_git/pull/37
please dont use separate views for stuff like `link_to`. Use helpers instead