Allow CommitTask daemon to recover from deleted repositories

Summary: If a user partially discovers a repository and then deletes it, the
timeline will have events from the old repository which this daemon won't be
able to parse.

Test Plan: @ajtrichards, can you apply this locally and restart your daemons
(phd stop, then relaunch them) and let me know if it fixes the issue?

Reviewers: ajtrichards, jungejason, tuomaspelkonen, aran

Reviewed By: ajtrichards

CC: aran, epriestley, ajtrichards

Differential Revision: 845