If directory provided to readDir is empty still fire async callback

[neilgoodman]

I ran into an issue when using readDir() on an empty directory: callback is never fired. This hangs my server.

To reproduce:

var git = require('git-fs');

git(__dirname);

git.readDir('fs', 'empty', function () {
    console.log('Works!');
});

This pull request contains a simple fix.

[neilgoodman]

Now looking at this it kind of doesn't make sense, because you can't have an empty directory in GIT. You can run into this issue during development, though, before you do any actual commits to the repo you are testing on.

So this pull request probably only has limited usefulness.