Fix error in D5634

Summary:
The original code seemed to assume the last level of edges was the destination edge, when it was an array under the destination edge key.
Using `array_keys` fixes the crashes that resulted when commits had tasks, projects or revisions attached.

Test Plan: Open up a commit with a revision attached.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, vrana

Differential Revision: https://secure.phabricator.com/D5698