tests: add a util module for sharing code

We've started duplicating code among test modules.  Start a common
utils module to hold that, and migrate over TempGitTree to start.

Change-Id: I10b2abd133535c90fbda4d6686602d7e5861d875
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/559041
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

tests/utils_for_test.py

@@ -0,0 +1,53 @@
+# Copyright (C) 2026 The Android Open Source Project
+#
+# 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.
+
+"""Various utility code used by tests.
+
+If you want to write a per-test fixture, see conftest.py instead.
+"""
+
+import contextlib
+from pathlib import Path
+import subprocess
+import tempfile
+from typing import Union
+
+import git_command
+
+
+def init_git_tree(path: Union[str, Path]) -> None:
+    """Initialize `path` as a new git repo."""
+    with contextlib.ExitStack() as stack:
+        # Tests need to assume, that main is default branch at init,
+        # which is not supported in config until 2.28.
+        cmd = ["git", "init"]
+        if git_command.git_require((2, 28, 0)):
+            cmd += ["--initial-branch=main"]
+        else:
+            # Use template dir for init.
+            templatedir = stack.enter_context(
+                tempfile.mkdtemp(prefix="git-template")
+            )
+            (Path(templatedir) / "HEAD").write_text("ref: refs/heads/main\n")
+            cmd += ["--template", templatedir]
+        cmd += [path]
+        subprocess.run(cmd, check=True)
+
+
+@contextlib.contextmanager
+def TempGitTree():
+    """Create a new empty git checkout for testing."""
+    with tempfile.TemporaryDirectory(prefix="repo-tests") as tempdir:
+        init_git_tree(tempdir)
+        yield tempdir