Fix GitHub Actions workflow bugs

* Remove `--track` from manual runs.
* Add missing candidate/reference for `push` event

Author: rcabell

.github/workflows/test-pr.yml

@@ -21,8 +21,8 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - name: Checkout candidate (automatic)
-        if: ${{ github.event_name == 'pull_request' }}
+      - name: Checkout candidate (pull request / push)
+        if: ${{ github.event_name == 'pull_request' || github.event_name == 'push' }}
         uses: actions/checkout@v2
         with:
           path: candidate
@@ -33,18 +33,25 @@ jobs:
           GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
         run: gh repo clone ${{ github.repository }} candidate && cd candidate && gh pr checkout -R ${{ github.repository }} ${{ github.event.inputs.pr }}
 
-      - name: Checkout reference (automatic)
+      - name: Checkout reference (pull request)
         if: ${{ github.event_name == 'pull_request' }}
         uses: actions/checkout@v2
         with:
           ref: ${{ github.event.pull_request.base.ref }}
           path: reference
 
+      - name: Checkout reference (push)
+        if: ${{ github.event_name == 'push' }}
+        uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.before }}
+          path: reference
+          
       - name: Checkout reference (manual)
         if: ${{ github.event_name == 'workflow_dispatch' }}
         env: 
           GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
-        run: gh repo clone ${{ github.repository }} reference && cd reference && git checkout --track origin/$(gh pr view ${{ github.event.inputs.pr }} --json baseRefName --jq '.baseRefName')
+        run: gh repo clone ${{ github.repository }} reference && cd reference && git checkout origin/$(gh pr view ${{ github.event.inputs.pr }} --json baseRefName --jq '.baseRefName')
 
       - name: Run testing container
         run: |