R1:5f797b4d478b

Recover from repositories becoming unpullable

Test Plan: If a repository's tracking flag is toggled while it's in queue, we may loop indefinitely.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11296