chore: add unit tests to @commitlint

Add a unit test showing the previous change works as expected.

Author: jeremysprofile

@commitlint/top-level/package.json

@@ -36,6 +36,7 @@
   },
   "license": "MIT",
   "devDependencies": {
+    "@commitlint/test": "^20.0.0",
     "@commitlint/utils": "^20.0.0"
   },
   "dependencies": {

@commitlint/top-level/src/index.test.ts

@@ -0,0 +1,64 @@
+import { test, expect } from "vitest";
+import fs from "fs/promises";
+import path from "node:path";
+import os from "node:os";
+import { git } from "@commitlint/test";
+
+import toplevel from "./index.js";
+
+test("should find git toplevel from repo root", async () => {
+	const cwd = await git.bootstrap();
+	const result = await toplevel(cwd);
+	expect(result).toBe(cwd);
+});
+
+test("should find git toplevel from subdirectory", async () => {
+	const cwd = await git.bootstrap();
+	const subdir = path.join(cwd, "subdir");
+	await fs.mkdir(subdir);
+
+	const result = await toplevel(subdir);
+	expect(result).toBe(cwd);
+});
+
+test("should prefer closer .git directory over farther .git file", async () => {
+	// Create a parent directory with a .git file (simulating dotfiles bare repo)
+	const parentDir = await fs.mkdtemp(path.join(os.tmpdir(), "parent-"));
+	const gitFileContent = `gitdir: ${path.join(parentDir, ".rcfiles")}`;
+	await fs.writeFile(path.join(parentDir, ".git"), gitFileContent);
+	await fs.mkdir(path.join(parentDir, ".rcfiles"));
+
+	// Create a child git repo inside the parent
+	const childDir = path.join(parentDir, "child-repo");
+	await fs.mkdir(childDir);
+	await fs.mkdir(path.join(childDir, ".git"));
+
+	// The toplevel should be the child repo, not the parent
+	const result = await toplevel(childDir);
+	expect(result).toBe(childDir);
+
+	// Cleanup
+	await fs.rm(parentDir, { recursive: true });
+});
+
+test("should handle .git file (submodule) correctly", async () => {
+	const cwd = await git.bootstrap();
+
+	// Create a submodule-like structure with a .git file
+	const submoduleDir = path.join(cwd, "submodule");
+	await fs.mkdir(submoduleDir);
+	const gitFileContent = `gitdir: ${path.join(cwd, ".git", "modules", "submodule")}`;
+	await fs.writeFile(path.join(submoduleDir, ".git"), gitFileContent);
+
+	const result = await toplevel(submoduleDir);
+	expect(result).toBe(submoduleDir);
+});
+
+test("should return undefined when no .git found", async () => {
+	const tmpDir = await fs.mkdtemp(path.join(os.tmpdir(), "no-git-"));
+
+	const result = await toplevel(tmpDir);
+	expect(result).toBeUndefined();
+
+	await fs.rm(tmpDir, { recursive: true });
+});