DX-3084 Update git diff logic (#910)

Author: ajrice6713

.github/workflows/listener.yml

@@ -72,10 +72,10 @@ jobs:
       - name: Check Diff
         run: |
           echo "========== modified files =========="
-          git diff --name-only HEAD^ HEAD
+          git diff --name-only ${{ github.base_ref }}
 
           echo "========== check paths of modified files =========="
-          git diff --name-only HEAD^ HEAD > files.txt
+          git diff --name-only ${{ github.base_ref }} > files.txt
           while IFS= read -r file
           do
             if [[ $file != site/specs/* ]]; then