Fix problem prefixBase option adds prefix to absolute paths incorrectly

When running against the options like
```
{
  src: ['/absolute/path/to/**/deep*.txt'],
  srcBase: 'deep', 
  prefixBase: true
}
```
The return paths will be prefixed with 'deep' incorrectly like:
` deep/absolute/path/to/**/deep*.txt`

This pull request is addressed to solve this problem with test case attached