get PR head SHA

Author: lobsterkatie

.github/workflows/build.yml

@@ -47,13 +47,15 @@ jobs:
         uses: actions/checkout@v2
         with:
           ref: ${{ env.HEAD_COMMIT }}
-          fetch-depth: 5
+          fetch-depth: 2
+      - name: Get PR head SHA
+        run: echo "HEAD_SHA=${{ github.event.pull_request.head.sha }}" >> $GITHUB_ENV
       - name: Setup tmate session
         uses: mxschmitt/action-tmate@v3
       - name: Get commit message
         run: |
           git rev-parse $GITHUB_HEAD_REF
-          echo "COMMIT_MESSAGE=$(git log -n 1 --pretty=format:%s $(git rev-parse $GITHUB_HEAD_REF))" >> $GITHUB_ENV
+          echo "COMMIT_MESSAGE=$(git log -n 1 --pretty=format:%s ${{ github.event.pull_request.head.sha }})" >> $GITHUB_ENV
       - name: "BRANCH: ${{ github.ref_name }}"
         run: echo ""
       - name: "COMMIT: ${{ env.COMMIT_MESSAGE }}"