Diffusion - fix bug from D5883

Summary: forgot to specify the commit! On github as https://github.com/facebook/phabricator/pull/318, though this is a tighter fix.

Test Plan: tried to save an owners package with a folder - FAIL - then with the patch - GREAT SUCCESS

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5933