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").
• Hide the edges from mail.

However, we'll sort-of lose the "revisionMatchData" explanation thing if I do that. Maybe this is fine? But when commits match because hashes match, it legitimately isn't obvious.

For now, just reduce the amount of harm/badness here.

Test Plan:

• Ran bin/repository reparse --publish ....
• Ran a Herald "Audit" rule using the "Accepted Differential revision" field.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13277

Differential Revision: https://secure.phabricator.com/D20468