R1:b2e91d220539

Move the "container updated" message for Buildables that build Diffs outside of the transaction

Summary:
Ref T13216. See PHI970. Ref T13054. See some discussion in T13216.

When a Harbormaster Buildable object is first created for a Diff, it has no `containerPHID` since the revision has not yet been created.

We later (after creating a revision) send the Buildable a message telling it that we've added a container and it should re-link the container object.

Currently, we send this message in…