R1:77eae81e1a5c

Policy - fix up DifferentialChangesetParser

Summary:
Ref T7094. We should do a policy query on the files IMO because there exists a scenario where the file gets locked down directly. This requires being a bit more disciplined about setting user, which in turn requires deciding whether or not to show edit / reply links as a separate piece of logic, not conditional on user presence.

This is not the best code but I don't think it gets worse with this and is just some other nuance in any larger cleanup we…