Run import sorting tests in a temporary Git repo

This fixes test failures when running tests from a cwd which isn't in a
Git repository.

src/darker/tests/test_import_sorting.py

@@ -12,6 +12,7 @@
 from darker.git import EditedLinenumsDiffer
 from darker.tests.helpers import isort_present
 from darkgraylib.git import RevisionRange
+from darkgraylib.testtools.git_repo_plugin import GitRepoFixture
 from darkgraylib.utils import TextDocument, joinlines
 
 ORIGINAL_SOURCE = ("import sys", "import os", "", "print(42)")
@@ -171,7 +172,7 @@ def test_isort_config(monkeypatch, tmpdir, line_length, settings_file, expect):
     config=None,
     line_length=None,
 )
-def test_build_isort_args(src, config, line_length, expect):
+def test_build_isort_args(git_repo: GitRepoFixture, src, config, line_length, expect):
     """``_build_isort_args`` returns correct arguments for isort"""
     result = darker.import_sorting._build_isort_args(src, config, line_length)