Create merge request view

Updated the create merge request view to use the horizontal form layout, and added tabs for commits and changes.
Also updated commits layout to use the same as the projects commits layout.

Screenshots: [Before](https://gitlab.com/uploads/stephanvl/gitlab-ce/892d1bf4c0/Screenshot_from_2014-12-29_20_33_19.png) - [After](https://gitlab.com/uploads/stephanvl/gitlab-ce/508b3100c7/Screenshot_from_2014-12-29_20_34_40.png)

Fixes #923

See merge request !270