Omit --initial-branch=master on old Git versions

CHANGES.rst

@@ -13,6 +13,7 @@ Fixed
   Darker's own code base.
 - Fix compatibility with ``black-22.10.1.dev19+gffaaf48`` and later – an argument was
   replaced in ``black.files.gen_python_files()``.
+- Fix tests to work with Git older than version 2.28.x.
 
 
 1.6.0_ - 2022-12-19

src/darker/git.py

@@ -9,7 +9,18 @@
 from functools import lru_cache
 from pathlib import Path
 from subprocess import DEVNULL, PIPE, CalledProcessError, check_output, run  # nosec
-from typing import Dict, Iterable, List, Optional, Set, Tuple, Union, overload
+from typing import (
+    Dict,
+    Iterable,
+    List,
+    Match,
+    Optional,
+    Set,
+    Tuple,
+    Union,
+    cast,
+    overload,
+)
 
 from darker.diff import diff_and_get_opcodes, opcodes_to_edit_linenums
 from darker.multiline_strings import get_multiline_string_ranges
@@ -33,6 +44,28 @@
 PRE_COMMIT_FROM_TO_REFS = ":PRE-COMMIT:"
 
 
+def git_get_version() -> Tuple[int, ...]:
+    """Return the Git version as a tuple of integers
+
+    Ignores any suffixes to the dot-separated parts of the version string.
+
+    :return: The version number of Git installed on the system
+    :raise: ``RuntimeError`` if unable to parse the Git version
+
+    """
+    output_lines = _git_check_output_lines(["--version"], Path("."))
+    version_string = output_lines[0].rsplit(None, 1)[-1]
+    # The version string might be e.g.
+    # - "2.39.0.windows.1"
+    # - "2.36.2"
+    part_matches = [re.match(r"\d+", part) for part in version_string.split(".")][:3]
+    if all(part_matches):
+        return tuple(
+            int(match.group(0)) for match in cast(List[Match[str]], part_matches)
+        )
+    raise RuntimeError(f"Unable to parse Git version: {output_lines!r}")
+
+
 def git_is_repository(path: Path) -> bool:
     """Return ``True`` if ``path`` is inside a Git working tree"""
     try:

src/darker/tests/conftest.py

@@ -9,7 +9,7 @@
 import pytest
 from black import find_project_root as black_find_project_root
 
-from darker.git import _git_check_output_lines
+from darker.git import _git_check_output_lines, git_get_version
 
 
 class GitRepoFixture:
@@ -27,7 +27,10 @@ def create_repository(cls, root: Path) -> "GitRepoFixture":
         env = {"HOME": str(root), "LC_ALL": "C", "PATH": os.environ["PATH"]}
         instance = cls(root, env)
         # pylint: disable=protected-access
-        instance._run("init", "--initial-branch=master")
+        force_master = (
+            ["--initial-branch=master"] if git_get_version() >= (2, 28) else []
+        )
+        instance._run("init", *force_master)
         instance._run("config", "user.email", "[email protected]")
         instance._run("config", "user.name", "CI system")
         return instance