[Finder] fix xargs pipe to work with spaces in dir names

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

.. otherwise grep fails:

````
grep: : No such file or directory
grep: /Users/havvg/Web: No such file or directory
grep: Development/symfony2/src/Symfony/Component/Finder/Tests/Fixtures/dolor.txt: No such file or directory
```