Merge trains can fail due to pipeline running in a refreshed merge ref

Description taken from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14495:

While we were QAing on staging server, we found that there is a race condition on merge trains with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29569 change.

Given MR1 and MR2 are just pushed to the merge train, it will be processed as

1. MR1 generates refs/merge-requests/1/merge and create a pipeline via MergeTrains::CreatePipelineService(i.e. MergeRequests::MergeToRefService)
2. Pipeline on MR1 finished. MR1 is merged into master.
3. MR2 generates refs/merge-requests/2/merge and create a pipeline via MergeTrains::CreatePipelineService(i.e. MergeRequests::MergeToRefService)
4. Since MR2's merge status will be reset. MergeRequest::MergeabilityCheckService will regenerate merge ref to refs/merge-requests/2/merge. This SHA is different from the above SHA which was used for the pipeline.sha.
5. Pipeline fails because it cannot find the SHA in the ref.

To combat this problem, Merge Train should generate a merge commit at refs/merge. This is already on our roadmap https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14296, but since we cannot wait until it has been deployed to prd, we're just implementing a part of the functionality earlier as patch.

After this fix, the above flow will be:

1. MR1 generates refs/merge-requests/1/train and create a pipeline via MergeTrains::CreatePipelineService(i.e. MergeRequests::MergeToRefService)
2. Pipeline on MR1 finished. MR1 is merged into master.
3. MR2 generates refs/merge-requests/2/train and create a pipeline via MergeTrains::CreatePipelineService(i.e. MergeRequests::MergeToRefService)
4. Since MR2's merge status will be reset. MergeRequest::MergeabilityCheckService will regenerate merge ref to refs/merge-requests/2/merge. This SHA is different from the above SHA which was used for the pipeline.sha.
5. Pipeline fails because it cannot find the SHA in the ref.