Ignore post-write repository synchronization if no devices are configured

Summary: Fixes T10789. If we aren't configured with a device, we never grabbed a lock in the first place, and should not expect one to be held.

Test Plan: Pushed non-cluster-configured Git SSH repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10789

Differential Revision: https://secure.phabricator.com/D15692