fix: load and copy config module without mutation

Author: jfinity

@commitlint/load/src/index.js

@@ -24,7 +24,7 @@ export default async (seed = {}, options = {cwd: process.cwd()}) => {
 	const base = loaded.filepath ? path.dirname(loaded.filepath) : options.cwd;
 
 	// Merge passed config with file based options
-	const config = valid(merge(loaded.config, seed));
+	const config = valid(merge({}, loaded.config, seed));
 	const opts = merge(
 		{extends: [], rules: {}, formatter: '@commitlint/format'},
 		pick(config, 'extends', 'plugins', 'ignores', 'defaultIgnores')

@commitlint/load/src/index.test.js

@@ -308,3 +308,15 @@ test('returns formatter name when unable to resolve from config directory', asyn
 		rules: {}
 	});
 });
+
+test('does not mutate config module reference', async t => {
+	const file = 'config/commitlint.config.js';
+	const cwd = await git.bootstrap('fixtures/specify-config-file');
+
+	const configPath = path.join(cwd, file);
+	const before = JSON.stringify(require(configPath));
+	await load({arbitraryField: true}, {cwd, file});
+	const after = JSON.stringify(require(configPath));
+
+	t.is(before, after);
+});