Don't attempt to discover parents commits for untracked branchs.

Summary: Fixes T5195. Currently, the `./bin/repository parents` workflow doesn't respect tracked branches and will attempt to build parents caches for all branches.

Test Plan: For at least one of our repositories, this patch fixes the `Unknown commit` exception. Unfortunately, it doesn't seem to completely solve this problem though, but I suspect that this is due to commits that were overwritten with a `git push --force` or similar.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5195

Differential Revision: https://secure.phabricator.com/D9322