Fix bugs in MergeWhenSucceeded
- This fixes support for merge when succeeded for statuses without ref.
- This fixes support for merge when succeeded for multiple stages. Stages are created after all builds for previous one are finished.
Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/9060 https://gitlab.com/gitlab-org/gitlab-ce/issues/8108 https://gitlab.com/gitlab-org/gitlab-ce/issues/12931 https://gitlab.com/gitlab-org/gitlab-ce/issues/13269