R1:20abf37b88cd

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…