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