Try to solve git: Not a valid object name HEAD^

.github/workflows/test-working-directory.yml

@@ -16,9 +16,10 @@ jobs:
           git config user.email "[email protected]"
           git config user.name "Test User"
           git commit --allow-empty -m "Initial empty commit"
-          echo 'def hello():    return "Hello, World!"' > test.py
+          echo 'def hello(): return "Hello, World!"' > test.py
           git add test.py
           git commit -m "Add test.py file"
+          echo 'def  hello():    return  "Hello, World!"' > test.py
 
       - name: Run Darker without working-directory (should not reformat)
         uses: ./
@@ -27,6 +28,7 @@ jobs:
         with:
           options: --check --diff -vvv
           src: test.py
+          revision: HEAD
 
       - name: Check if file was not reformatted
         run: |
@@ -41,6 +43,7 @@ jobs:
           options: --check --diff -vvv
           src: test.py
           working-directory: ${{ runner.temp }}/test-repo
+          revision: HEAD
 
       - name: Verify file was reformatted
         run: |
@@ -60,6 +63,7 @@ jobs:
           options: --check --diff -vvv
           src: test.py
           working-directory: ${{ runner.temp }}/non-existent-dir
+          revision: HEAD
 
       - name: Check if file was not reformatted with incorrect working-directory
         run: |