Add test to ensure that we always create repo with branch master

We also add a fixture which creates a predictable empty gitconfig for
the tests.

tests/conftest.py

@@ -5,6 +5,8 @@
 """
 from __future__ import annotations
 
+import os
+
 from pathlib import Path
 from typing import Protocol
 
@@ -23,6 +25,23 @@ def __call__(self, args: list[str]) -> Result:
         ...
 
 
+@pytest.fixture(autouse=True)
+def gitconfig(tmp_path: Path) -> Path:
+    """Ensure that tests have a predictable empty gitconfig.
+
+    We set autouse=True, so that the fixture is automatically used for all
+    tests.  Tests that want to access the mock gitconfig can explicitly specify
+    the fixutre, so they get the path to the mock gitconfig.
+    """
+    os.environ["GIT_CONFIG_NOSYSTEM"] = "true"
+    os.environ["HOME"] = str(tmp_path)
+    os.environ["XDG_CONFIG_HOME"] = str(tmp_path / ".config")
+    gitconfig = tmp_path / ".config" / "git" / "config"
+    os.makedirs(gitconfig.parent, exist_ok=True)
+
+    return gitconfig
+
+
 @pytest.fixture
 def cli_runner() -> RunnerFunc:
     r = CliRunner()

tests/test_gitrepo.py

@@ -752,3 +752,14 @@ def test_gitrepo_is_ahead_of_remote_local_branch(tmp_path: Path):
     # verify that our local branch is ahead of both local and remote tracking master
     assert len(list(r.repo.iter_commits("master..local"))) == 1
     assert len(list(r.repo.iter_commits("origin/master..local"))) == 1
+
+
+def test_gitrepo_init_always_master(gitconfig: Path, tmp_path: Path):
+    cfg = git.config.GitConfigParser(file_or_files=gitconfig, read_only=False)
+    cfg.add_value("init", "defaultBranch", "main").write()
+
+    r = gitrepo.GitRepo(None, tmp_path / "repo.git")
+    r.commit("Initial commit")
+
+    assert len(r._repo.heads) == 1
+    assert r._repo.heads[0].name == "master"