fix git diff branch

Author: reingart

.github/workflows/python-app.yml

@@ -30,7 +30,7 @@ jobs:
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Test with pytest
       run: |
-        git diff origin/main --name-only -- practice | xargs dirname | sort | uniq | xargs pytest
+        git diff origin/main HEAD --name-only -- practice | xargs dirname | sort | uniq | xargs pytest
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names