R1:ef05bf335d8e

Allow Harbormaster builds to publish to a different object

Summary:
Fixes T9276. Fixes T8650. The story so far:

- We once published build updates to Revisions.
- An unrelated fix (D10911) sent them to the Diffs instead of Revisions, which isn't useful, since you can't see a diff's timeline anywhere.
- This also caused a race condition, where the RevisionEditor and DiffEditor would update the diff simultaneously (T8650).
- The diff update was just disabled to avoid the race (part of D13441).