ci: improve backend change detection

Author: pablogsal

.github/workflows/ci.yml

@@ -52,7 +52,9 @@ jobs:
         id: changes
         run: |
           if [ "${{ github.event_name }}" = "push" ]; then
-            CHANGED=$(git diff --name-only HEAD~1 HEAD 2>/dev/null || echo "backend/")
+            CHANGED=$(git diff --name-only ${{ github.event.before }} ${{ github.sha }} 2>/dev/null \
+              || git diff --name-only HEAD~1 HEAD 2>/dev/null \
+              || echo "backend/")
           else
             CHANGED=$(git diff --name-only ${{ github.event.pull_request.base.sha }} ${{ github.event.pull_request.head.sha }})
           fi