specify diff between main and PR branch

based on this: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-comparing-branches-in-pull-requests#about-three-dot-comparison-on-github

and this: github/docs#32224 (comment)

.github/workflows/schema-convert-ci.yml

@@ -60,7 +60,7 @@ jobs:
           repo-token: ${{ secrets.GITHUB_TOKEN }}
 
       - name: List changed attributes
-        run: git diff --name-only > ./tests/changed-files.txt
+        run: git diff --name-only --merge-base origin/main ${ github.ref_name } > ./tests/changed-files.txt
 
       - name: Identify changed manifests
         working-directory: tests