Only load refs that are actual commits

Summary: Fix T11301. Git is git.

Test Plan: tagged a file! run discover. no crash.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T11301

Differential Revision: https://secure.phabricator.com/D16261