R1:d1223ac577e8

When a commit appears as an ancestor of a permanent ref for the first time, run all import steps

Summary:
Depends on D20427. Ref T13277. As an optimization, when we discover that a commit which was previously only on a non-permanent ref ("tmp-epriestley-123") is now reachable from a permanent ref ("master"), we currently queue only a new "message" parse step.

This is an optimization because these commits previously got the full treatment (feed, publish, audit, etc) as soon as they were discovered. Now…