ci: Check the style of commit messages

Check all commits, not just the latest one, as a branch may consist of multiple commits. Exclude commits from a known old master (which may contain issues which we want to error out for on new commits).

This uses a hardcoded commit hash from master (which can be bumped later to speed up later testing), as the CI environment doesn't know what the state is for the master branch of the main public repo, only of the current repo.