Fix a lookup issue in Owners

Summary:
Fixes T4477. Sort of winging this but it's probably the right fix?

One error in T4477.

One error via email:

```
[2014-04-15 17:44:34] ERROR 8: Undefined index: /some_index/ at [/phab_path/phabricator/src/applications/owners/storage/PhabricatorOwnersPackage.php:213]
  #0 PhabricatorOwnersPackage::findLongestPathsPerPackage(Array of size 3 starting with: { 0 => Array of size 3 starting with: { id => 5 } }, Array of size 8 starting with: { / => Array of size 2 starting with: { /some_index/some_file.py => true } }) called at [/phab_path/phabricator/src/applications/owners/storage/PhabricatorOwnersPackage.php:170]
  #1 PhabricatorOwnersPackage::loadPackagesForPaths(Object PhabricatorRepository, Array of size 2 starting with: { 0 => /some_index/some_file.py }) called at [/phab_path/phabricator/src/applications/owners/storage/PhabricatorOwnersPackage.php:119]
...
```

Test Plan: Will make @zeeg do it.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, zeeg

Maniphest Tasks: T4477

Differential Revision: https://secure.phabricator.com/D8779