fix xargs pipe to work with spaces in dir names
merged branch havvg/hotfix/finder-spaces-in-dir (PR #7327)

This PR was merged into the 2.2 branch.

Commits
-------

a735cbd fix xargs pipe to work with spaces in dir names

Discussion
----------

[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
```