Use merge_commit_sha when checking out closed pull requests

Author: Jackenmen

action.yaml

@@ -12,12 +12,20 @@ runs:
     - name: Checkout the base branch for the pull request.
       uses: actions/checkout@v3
 
-    - name: Checkout the merge branch for the pull request.
+    - name: Checkout the merge branch for the open pull request.
+      if: github.event.pull_request.state == 'open'
       run: |-
         git fetch origin +refs/pull/${{ github.event.pull_request.number }}/merge
         git checkout FETCH_HEAD
       shell: bash
 
+    - name: Checkout the merge branch for the closed pull request.
+      if: github.event.pull_request.state != 'open'
+      run: |-
+        git fetch origin ${{ github.event.pull_request.merge_commit_sha }}
+        git checkout FETCH_HEAD
+      shell: bash
+
     - name: Setup Python for the action.
       id: python-for-action
       uses: actions/setup-python@v4