test fix

Author: anandmukul93

.github/workflows/backport-assistant-prerun.yml

@@ -78,7 +78,8 @@ jobs:
         id: check_merges
         if: needs.get-pr-info.outputs.has_no_backport == 'false'
         run: |
-          MERGE_COMMITS=$(git log --merges --oneline origin/${{ github.base_ref }}..HEAD)
+          BASE_REF="${{ github.event.pull_request.base.ref }}"
+          MERGE_COMMITS=$(git log --merges --oneline origin/${{ BASE_REF }}..HEAD)
           if [ -n "$MERGE_COMMITS" ]; then
             echo "::error::Merge commits found."
             echo "merge_commit_found=true" >> $GITHUB_OUTPUT