Detect when merge train ref includes legacy commit message

Problem to solve

During and after an upgrade to a version with fast-forward merge train support, there can be existing merge train refs with legacy commit messages. They should NEVER be merged into the target branch.

Background

In the upcoming fast-forward merge trains, we merge via a fast-forward merge from the merge train ref instead of re-performing the merge from the source branch (which is what the traditional merge commit method does). The problem with that is that merge train refs are currently constructed with a commit message as follows (source):

def commit_message(merge_request, previous_ref)
  "Merge branch #{merge_request.source_branch} with #{previous_ref} " \
  "into #{merge_request.train_ref_path}"
end

Once we have fully rolled out #420161 (closed), the merge train ref will be representative for any new merge train. But this does not apply to merge train refs that have already been created:

• In an upgrade with downtime: There may be an existing train that gets picked up by the new version.
• In a zero-downtime upgrade (e.g. GitLab.com): New train refs may get touched by old code and vice versa.

Proposal

When merging a merge train, if a legacy commit is detected in the train ref, fall back to the legacy merge strategy.

Outline:

1. When constructing the train ref for a given car, check whether the the previous car is fast-forward compatible:
   • If it is, then construct the given car as fast-forward compatible as well
   • Otherwise, fall back to the legacy strategy
2. When picking a merge strategy for a mergeable car, check whether the train ref is fast-forward compatible.
   • If it is, attempt the fast-forward.
   • If not, fall back to the legacy strategy

Now, the detection for a given car works as follows:

1. Compare the car's commit SHA against merge_request.merge_params.dig('train_ref', 'commit_sha'). The latter will only be recorded when standard commit messages are enabled.
2. Compare the car's commit message against the legacy commit message.