By default actions/checkout@v2 only checks out the latest commit of the current branch. Unfortunately, this causes the `git diff` check in `check-pr.sh` to fail, as `master` branch isn't available`.
This change checks out all commits of all branches, which allows `git diff` to work correctly.
Ideally, I'd like to use the `ref` setting of actions/checkout@v2, but that only supports a single ref so it's not possible to checkout the PR branch _and_ `master` branch at the same time.
