Create the test repos with --initial-branch=master

git prints a warning when creating a new repository if
`init.defaultBranch` is not configured and `git init` is called without
`--initial-branch`.

Furthermore, several tests expect the initial branch to be `master`.
Those tests will fail if the default configuration for the default
branch ever changes.

Therefore, the repositories created for testing are explicitly
configured to use the initial branch `master`.

Author: gdiscry

src/darker/tests/conftest.py

@@ -25,7 +25,7 @@ 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")
+        instance._run("init", "--initial-branch=master")
         instance._run("config", "user.email", "[email protected]")
         instance._run("config", "user.name", "CI system")
         return instance