Do not show "merge blocked" message when a fast-forward merge train is running for current source SHA

Problem to solve

When a merge multi-car fast-forward merge train is running and the first car gets merged, subsequent cars' MRs will show a "Merge blocked" message prompting for a rebase, when one is in fact not necessary. Example from staging with fast_forward_merge_trains_support enabled:

Proposal

Don't ask for a rebase when the source SHA for the merge train car's pipeline matches the current source SHA.