Fixing issue when running 'arc diff'

Summary:
There's a crash when `arc diff` is running it's lint / unit test
phase which describes that this function should be public instead
of private:

```Fatal error: Access level to PhabricatorPholioMockTestDataGenerator::loadPhabrictorUserPHID() must be public (as in class PhabricatorTestDataGenerator) in /srv/www/tychaia.com/phab/phabricator/src/applications/pholio/lipsum/PhabricatorPholioMockTestDataGenerator.php on line 102```

Test Plan: `arc diff` should not crash.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5828