fix,test: run stdout tests in a temporary repo

src/darker/tests/test_main.py

@@ -946,15 +946,18 @@ def test_stdout_path_resolution(git_repo, capsys):
 
 
 @pytest.mark.parametrize("newline", ["\n", "\r\n"], ids=["unix", "windows"])
-def test_stdout_newlines(tmp_path, monkeypatch, capsysbinary, newline):
+def test_stdout_newlines(git_repo, monkeypatch, capsysbinary, newline):
     """When using ``--stdout``, newlines are not duplicated.
 
     See: https://github.com/akaihola/darker/issues/604
 
+    The `git_repo` fixture is used to ensure that the test doesn't run in the Darker
+    repository clone in CI. It helps avoid the Git error message
+    "fatal: Not a valid object name origin/master" in the NixOS CI tests.
+
     """
     if WINDOWS and sys.version_info < (3, 10):
         # See https://bugs.python.org/issue38671
-        monkeypatch.chdir(tmp_path)
         Path("new-file.py").touch()
     code = f"import collections{newline}import sys{newline}".encode()
     with patch("sys.stdin.buffer.read", return_value=code):
@@ -968,15 +971,18 @@ def test_stdout_newlines(tmp_path, monkeypatch, capsysbinary, newline):
 
 
 @pytest.mark.parametrize("newline", ["\n", "\r\n"], ids=["unix", "windows"])
-def test_stdout_newlines_subprocess(tmp_path, monkeypatch, newline):
+def test_stdout_newlines_subprocess(git_repo, monkeypatch, newline):
     """When using ``--stdout``, newlines are not duplicated.
 
     See: https://github.com/akaihola/darker/issues/604
 
+    The `git_repo` fixture is used to ensure that the test doesn't run in the Darker
+    repository clone in CI. It helps avoid the Git error message
+    "fatal: Not a valid object name origin/master" in the NixOS CI tests.
+
     """
     if WINDOWS and sys.version_info < (3, 10):
         # See https://bugs.python.org/issue38671
-        monkeypatch.chdir(tmp_path)
         Path("new-file.py").touch()
     code = f"import collections{newline}import sys{newline}".encode()
     try: