Watch does not work for glob pattern '<dir>/**' if '<dir>' does not exist on fs

[eGavr]

Describe the bug

Files are not being watched for glob <dir>/** if <dir> does not exist.

Versions (please complete the following information):

• Chokidar version [2.0.4]
• Node version [8.6.0]
• OS version: [Ubuntu 16.04]

To Reproduce

// index.js
const chokidar = require('chokidar');
const watcher = chokidar.watch(['dir/**']);

watcher.on('add', path => console.log(path));

Run this code and create file.ext in dir:

mkdir dir && touch dir/file.ext

You will see nothing in console.

[paulmillr]

Duplicate of latter-created #872