R1:1a3ac099751b

Add "requestedObjectPHID" to ReleephRequest

Summary:
Ref T3551. Currently, ReleephRequests don't have a direct concept of the //object// being requested. You can request `D123`, but that is just a convenient way to write `rXyyyy`.

When the UI wants to display information about a revision, it deduces it by examining the commit.

This is primarily an attack on T3551, so we don't need to load <commit -> edge -> revision> (in an ad-hoc way) to get revisions. Instead, when you request a revision we keep track…