R1:6d36eb911309

Denormalize Diff PHIDs onto Revisions

Summary:
Ref T12539. See PHI190. Currently, each Diff has a `revisionID`, but Revisions do not point at the current active diff. To find the active diff for a given revision, we need to issue a separate query.

Furthermore, this query is inefficient for bulk loads: if we have a lot of revisions, we end up querying for all diff IDs for all those revisions first, then selecting the largest ones and querying again to get the actual diff objects. This strategy could…