Checkout all branches in GitHub Actions (#4437)

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.

.github/workflows/ci.yml

@@ -12,6 +12,8 @@ jobs:
 
     - name: Checkout
       uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
 
     - name: Set up PR environment
       if: github.event.number != null