Add a CI status check to prevent merging PRs that contain merges

[neilmayhew]

Closes #4307

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

Awesome! Thank you!

[lehins]

Looks like this git command does not work as expected: https://github.com/IntersectMBO/cardano-ledger/actions/runs/8886398544/job/24399775489?pr=4308

[neilmayhew]

This seems to be working now. It failed (for the right reasons!) on the last run when there was still a merge commit on the branch. If it passes this time, it should be good to go.

[lehins]

Perfect!