Don't mark merge request as updated when merge status relative to target branch changes.
Addresses https://gitlab.com/gitlab-org/gitlab-ce/issues/1254 and private issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2165.
Addresses https://gitlab.com/gitlab-org/gitlab-ce/issues/1254 and private issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2165.