Add "Check out branch" button to the MR page.

I appreciated the thought behind adding the "Fetch this branch using ..." line, but not so much the implementation. I think a "Check out branch" button alongside the existing "Download as" makes more sense to get instructions on how to get the changes on your local machine, since that's pretty much what "Download as" does as well. 

Like the "command line", the button opens the command line merge instructions modal. I've improved the text of that modal in !1120.

![Screen_Shot_2015-08-08_at_22.10.19](https://gitlab.com/gitlab-org/gitlab-ce/uploads/fd185df012e9759dfea198ed9fa3f80f/Screen_Shot_2015-08-08_at_22.10.19.png)

It looks even better in combination with !1121, which gets rid of the "If you want to ..." line:

![Screen_Shot_2015-08-08_at_22.10.55](https://gitlab.com/gitlab-org/gitlab-ce/uploads/3fa469d0e2bd4e3ec4992ffa5864c718/Screen_Shot_2015-08-08_at_22.10.55.png)

cc @dzaporozhets @rspeicher 

See merge request !1116