Simulating PR with Git diff call

[bearrito]

I feel like this should be a simple supported use-case, so I must be missing something

Simplifying:

I want to run this command as a step

git diff --raw "$GITHUB_BASE_REF" "$GITHUB_HEAD_REF" 

What set of events and command args do I need to make this happen?

My events.json

{
    "pull_request": {
      "head": {
        "ref": "refs/heads/feature/no-integration-test-readme"
      },
      "base": {
        "ref": "refs/heads/main"
      }
    }
}

Both of those refs exist locally.

I invoke like

act pull_request -e events.json -

My checkout is like this

 steps:
    - uses: actions/checkout@v4
      with:
        ref: ${{ github.event.pull_request.head.sha }}

I receive this error

[Tests/Integration Conditional]   ❓  ::group::Checking out the ref
| [command]/usr/bin/git checkout --progress --force refs/remotes/pull/%!f(<nil>)/merge
| Note: switching to 'refs/remotes/pull/%!f(<nil>)/merge'.
| 
| You are in 'detached HEAD' state. You can look around, make experimental
| changes and commit them, and you can discard any commits you make in this
| state without impacting any branches by switching back to a branch.
| 

I have debugging code

echo "BASE_REF: $GITHUB_BASE_REF"
        echo "REF: $GITHUB_REF"
        echo "HEAD REF: $GITHUB_HEAD_REF"
        echo "SHA: $GITHUB_SHA"

which prints

| BASE_REF: refs/heads/main
| REF: refs/pull/%!f(<nil>)/merge
| HEAD REF: refs/heads/feature/no-integration-test-readme
| SHA: 47d4f62fb3876ae30c95464ef2a4d427a552e98d