Record initial branch when recording commits

Summary:
We have a race condition right now, where we may insert a commit without seenOnBranches. This means shouldAutocloseCommit will return false (since it's not on any autoclose branches) if the message parser runs fast enough, causing it to associate the commit but not close the revision. This happened for D2851.

Also prompt the user to repair broken repositories.

Test Plan: Ran discovery / repair. Ran discovery on new commits. Verified 'seenOnBranches' value. Deleted some data, verified "repair" error. Repaired repository.

Reviewers: jungejason, nh, vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2858