Use "null", not "-1", as a local "no version" marker when performing intracluster repository sync
The synchronization log column is uint32? and -1 doesn't go into that column.
Since we're only using -1 for convenience to cheat through $max_version > $this_version checks, use null instead and make the checks more explicit.
Test Plan: Reproducing this is a bit tricky and I cheated fairly heavily to force the code down this pathway without actually building a multi-device cluster, but I did reproduce the original exception, apply the patch, and observe that it fixed things.
Reviewed By: amckinley
Maniphest Tasks: T13242
Differential Revision: https://secure.phabricator.com/D20047