fix: Isolate tests from the calling git process's environment

tests/conftest.py

@@ -18,12 +18,27 @@
 
 from collections.abc import Callable
 import json
+import os
 from pathlib import Path
 from typing import Any
 
 import pytest
 
 _FIXTURE_DIR = Path(__file__).resolve().parent / "fixtures"
+GIT_ENVIRONMENT_PREFIX = "GIT_"
+
+
+def pytest_configure(config: pytest.Config) -> None:
+    """Remove every ``GIT_*`` variable before any fixture or xdist worker starts.
+
+    A hook run by ``git commit -a`` exports an absolute ``GIT_INDEX_FILE`` that
+    points at this repository's ``index.lock``; a test that runs git in a
+    temporary repository would otherwise read and write that index instead.
+    """
+    for name in [
+        name for name in os.environ if name.startswith(GIT_ENVIRONMENT_PREFIX)
+    ]:
+        del os.environ[name]
 
 
 def _load_json_fixture(name: str) -> dict[str, Any]:

tests/test_git_environment.py

@@ -0,0 +1,24 @@
+# Copyright 2026 DataRobot, Inc. and its affiliates.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests run free of the environment a calling git process exports."""
+
+from __future__ import annotations
+
+import os
+
+
+def test_tests_see_no_git_environment_variables() -> None:
+    """A hook run by ``git commit -a`` exports ``GIT_INDEX_FILE``; tests never see it."""
+    assert [name for name in os.environ if name.startswith("GIT_")] == []