merge_requests: add fetch command to the MR page

In our instance, users asked for an easy way to get the contents of the MR into their local clone. This adds a command line which can be used to fetch the contents of the MR.

Screenshot:

![fetch-command](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d83acfbdb69f023d13b45dff5d583ac1/fetch-command.png)

See merge request !678