Rebase merge train refs when adding a car after skip-merge

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Problem to solve

When an immediate non-interrupting merge occurs to a project with a long-running merge train, the train ref starts to diverge from the target branch.

• A long running merge train can permanently diverge from the target branch, accumulating changes that make the merge train pipelines non-representative of what would run on the target branch.

Technical Proposal

• When adding a car to a train, if the predecessor is a skip-merge, then use previous_ref = rebase(previous_non_skip_merge.train_ref, target_ref) (pseudocode) when creating the train ref for the new car.