R1:09c6d42b9599

Mostly make blame work with DocumentEngine

Summary: Ref T13105. This needs refinement but blame sort of works again, now.

Test Plan: Viewed files in Diffusion and Files; saw blame in Diffusion when viewing in source mode.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19309