Use the realpath version of cwd

When the cwd is under a symlink, on Windows at least, `isPathCwd` and `isPathInCwd` fail because `process.cwd()` doesn't return a realpath version of the cwd:
```
> process.cwd();
'C:\\Users\\Gary\\Documents\\GitHub\\grunt'
> fs.realpathSync(process.cwd());
'd:\\Users\\Gary\\Documents\\GitHub\\grunt'
```