Follow-on to #259. This change helps prevent PRs that are not passing from unnecessarily being updated, but it also slows down updates from Bulldozer in general.

I wonder if Bulldozer could be configured to automatically add a trigger label after required statuses pass for the first time? This would help prevent PRs that are not passing from getting updates while still keeping passing PRs updating more quickly than waiting for each commit to fully complete required statuses.

Alternatively, a more complex process of Bulldozer adding and removing update trigger labels based on conditions could give Bulldozer somewhat of a queueing functionality, but it may be tricky to make this idempotent and not introduce flip-flopping in state.