Include full path of source and target branch names in New Merge Request page

The previous title on the New Merge Request page would only show the group name. which could be confusing. Before, if you attempted to create a new merge request within the repo, it might look like this:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/d300d3a362406628ffc7c8f2b5cc6b5d/image.png)

Since they are in the same repo, this MR just simplifies this to the branches:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/5d7b3efc2399b37f84ccfd42bd5d6e8d/image.png)

However, if you create a MR from a forked project, you now see the full namespace:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/7998c3d19e90d405f4634202cc7181c5/image.png)





Closes #2875

See merge request !1477