Fix bugs in MergeWhenSucceeded

1. This fixes support for merge when succeeded for statuses without ref.
2. 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

/cc @grzesiek @DouweM @rspeicher 


See merge request !2894