Respect merge method and squash preferences for merge train ref

What does this MR do and why?

Completes the rollout of #420161 (closed).

With this change, merge train ref creation respects the merge request's squash preferences, and the project's merge method preference. For fast-forward and semi-linear merges, an automatic rebase is performed for cars 2+ in the train. This is part of enabling fast-forward and semi-linear merges for merge trains (but does not enable it on its own).

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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