Carefully avoid the harbormaster/differential race

Summary: Ref T8650. This should stop the problem, but isn't a root cause fix. See discussion on the task.

Test Plan: Made some local diffs, but this is a bit hard to reproduce reliably.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8650

Differential Revision: https://secure.phabricator.com/D13441