Deduplicate parents from PhabricatorGitGraphStream

Summary: Fixes T5226. It's rare (but possible) for a commit to have the same parent more than once in Git.

Test Plan: Ran `bin/repository parents` on a normal repository.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5226

Differential Revision: https://secure.phabricator.com/D9344