Improve test_action_default and fix linux

mu_repo/tests/test_action_default.py

@@ -8,18 +8,23 @@
 def test_action_default(workdir, monkeypatch):
     workdir = Path(workdir)
     monkeypatch.chdir(workdir)
-    subprocess.check_call(f'git init core')
-    subprocess.check_call(f'git init app')
+    # Init two repos: core, and app, with app depending on core.
+    subprocess.check_call(['git', 'init', 'core'])
+    subprocess.check_call(['git', 'init', 'app'])
     (workdir/ 'app/.mu_repo').write_text("repo=.\nrepo=../core\n", encoding="utf-8")
 
+    # Add a new configuration value only to 'app'.
     monkeypatch.chdir(workdir/ 'app')
-    subprocess.check_call(f'git config --add foo.bar foo-value')
+    subprocess.check_call(['git', 'config', '--add', 'foo.bar', 'foo-value'])
 
-    subprocess.check_call(f'git config --get foo.bar')
+    # Sanity check we can get the option.
+    subprocess.check_call(['git', 'config', '--get', 'foo.bar'])
 
+    # Getting a standard option will work for all repositories.
     status = mu_repo.main(args=['config', '--get', 'core.bare'], config_file='.mu_repo')
     assert status == mu_repo.Status("Finished", succeeded=True)
 
+    # Getting 'foobar' will only work for 'app'.
     status = mu_repo.main(args=['config', '--get', 'foo.bar'], config_file='.mu_repo')
     assert status == mu_repo.Status("Failed:\n  ../core", succeeded=False)