split one test into three

(I am trying to debug the third one on the Mac, and this makes it easier.)

tests.py

@@ -40,7 +40,9 @@ def create_mock_git_repository(target_directory, create_directory=True):
         # that.
         user_details = "-c user.name='Tes Ta' -c user.email='tester@some-ci.org'"
         subprocess.run(
-            f'git {user_details} commit -m "first commit" readme.txt', shell=True, check=True
+            f'git {user_details} commit -m "first commit" readme.txt',
+            shell=True,
+            check=True,
         )