Use prototype to apply `concat` and comment intent

lib/index.js

@@ -82,7 +82,9 @@ export default class Now extends EventEmitter {
           forceSync,
           name: pkg.name || basename(path),
           description: pkg.description,
-          files: [].concat.apply([], Array.from(this._files).map(([sha, { data, names }]) => {
+          // Flatten the array to contain files to sync where each nested input
+          // array has a group of files with the same sha but different path
+          files: Array.prototype.concat.apply([], Array.from(this._files).map(([sha, { data, names }]) => {
             return names.map((name) => {
               return {
                 sha,