New scenario to ensure that a push updates an existing merge request
Part of https://gitlab.com/gitlab-org/gitlab-qa/issues/53.
Steps:
1. Setup a project
1. Create a merge request
1. Push a commit to the MR's branch
1. Visit the merge request page
1. Ensure that the commit appears in the `Commits` tab
1. Ensure that the commit's diff appears in the `Changes` tab
issue