scripts/patch-release-for-pr: fix version check point and keep diff points

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.github/workflows/automate_merge_message.yml

@@ -25,7 +25,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
         with:
-          ref: '${{ github.event.pull_request.head.sha }}'
+          ref: '${{ github.event.pull_request.merge_commit_sha }}'
 
       - name: fetch base
         run: git fetch --depth 1 origin ${{ github.event.pull_request.base.sha }}
@@ -36,7 +36,7 @@ jobs:
         run: |
           rm -f generate.js
           wget -O generate.js https://raw.githubusercontent.com/backstage/backstage/master/scripts/generate-merge-message.js 1>&2
-          node generate.js FETCH_HEAD > message.txt
+          node generate.js ${{ github.event.pull_request.base.sha }} ${{ github.event.pull_request.head.sha }} > message.txt
 
       - name: Post Message
         uses: actions/github-script@v6