fix(tests): deploy-range git helper ignores the hook's GIT_DIR/GIT_INDEX_FILE

The pre-commit hook runs the dev tests with GIT_DIR and GIT_INDEX_FILE set
to the real repo; the temp-repo helper inherited them, so every commit that
touched dev/ failed at 'git checkout -b feature' (exit 128) while the same
test passed from a plain shell.

tests/dev/test_gen_config_deploy.py

@@ -105,8 +105,11 @@ def test_image_jobs_gate_on_changes_outputs_not_head_commit():
 
 
 def _git(*args: str, cwd: Path) -> str:
+    # Drop GIT_DIR / GIT_INDEX_FILE / GIT_WORK_TREE: the pre-commit hook runs
+    # this file with them pointing at the real repo, which would redirect
+    # every call away from the temp repo under `cwd`.
     env = {
-        **os.environ,
+        **{k: v for k, v in os.environ.items() if not k.startswith("GIT_")},
         "GIT_AUTHOR_NAME": "t",
         "GIT_AUTHOR_EMAIL": "t@example.invalid",
         "GIT_COMMITTER_NAME": "t",