force `subprocess.run` to check return code is 0

In general, this should help to capture unintentional errors.

Where the change has created failures, they have been fixed. (Mostly because the git repository that was looked for was missing.)

tests.py

@@ -38,7 +38,9 @@ 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,
         )