R1:56f5c2443bc7

Consolidate readers of "differential.revisionID" property

Summary:
Depends on D20467. Ref T13277. Currently, the "MessageParserWorker" writes this property on commits, then Herald and Audit both read it.

Make them share code so this property has one writer and one reader. This property isn't great, but at least now the badness is hidden.

Currently, we can't just use edges because they may not have been written yet. I am likely to just do this, soon:

- Just write the edges (in "MessageParserWorker").