Display renamed files in diff views

Show both the old and new filenames when viewing the diff for a renamed file, as suggested on [feedback.gitlab.com](http://feedback.gitlab.com/forums/176466-general/suggestions/4130669-display-moved-files-with-a-single-line-instead-of).

This depends on a change to gitlab_git; file rename detection is enabled there in [merge request 6](https://gitlab.com/gitlab-org/gitlab_git/merge_requests/6) on gitlab.com.  However, this change shouldn't break the existing diff view in the meantime.