Use grunt root path when collecting git files

No reason to reinvent the wheel here. We already determine the root path
of the project in our grunt config, so we just use that instead.

Fixes #5781

tasks/collect_files_to_commit.js

@@ -1,9 +1,9 @@
 import filesToCommit from './utils/files_to_commit';
 import { resolve } from 'path';
 
-const root = resolve(__dirname, '..');
-
 export default function registerCollectFilesToCommit(grunt) {
+  const root = grunt.config.get('root');
+
   grunt.registerTask('collectFilesToCommit', function () {
     filesToCommit(root)
     .then(files => {