Only persist generated ref commits after merge

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Summary

Right now, we persist the generated ref commits inside the CreateRefService. We delete them, if something goes wrong during a merge train merge.

What we should do instead is try to create them only, when the merge was successful.

Improvements

Risks

Involved components

Optional: Intended side effects

Optional: Missing test coverage

Edited by 🤖 GitLab Bot 🤖