Verifying commit authors' email addresses

Hello!

I'd like to list all commits in a PR to check email addresses whether they are registered on GitHub.

```
git show --no-patch --pretty="format:%ae" \
  "${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }}
```

works only with `fetch-depth: 0`
https://github.com/szepeviktor/small-project/runs/769955284

Could you suggest an optimized alternative?
