Diff colors on MR Discussion like on Changes-tab

Fixes #2216

# Before
![Screen_Shot_2015-08-06_at_21.44.58](https://gitlab.com/gert/gitlab-ce/uploads/ed5935e0713dd58f9cd4b17a1fecaa86/Screen_Shot_2015-08-06_at_21.44.58.png)

# After
![Screen_Shot_2015-08-06_at_21.44.33](https://gitlab.com/gert/gitlab-ce/uploads/0816360afcc7cf860d2eadf8dd63b49e/Screen_Shot_2015-08-06_at_21.44.33.png)

See merge request !1108