grunt.config.merge getter differently

Grunt config merge does not work exactly in v1.0.2 and v1.0.3.
Second and third getters are differently.

Example:
```javascript
module.exports = function (grunt) {
"use strict";

var i = 0, y = 0;

grunt.initConfig({});

grunt.config.merge({
a: {
b: {
get inc () { return y++; }
},
get inc () { return i++; }
}
});

grunt.registerTask("default", function() {
console.debug( grunt.config.get("a.b.inc") );
console.debug( grunt.config.get("a.b.inc") );
console.debug( grunt.config.get("a.inc") );
console.debug( grunt.config.get("a.inc") );
});

};
```
```cmd
$ grunt --version
grunt v1.0.2
$ grunt
0
1
0
0
```

```cmd
$ grunt --version
grunt v1.0.3
$ grunt
0
0
0
0
```

Env:
```bash
$ npm -v
6.0.1
$ nodejs -v
v10.4.0
```