Make r+ by a delegatee infer the delegator

[jdm]

This will prevent a common source of incorrect historical metadata in the commit log.

[Manishearth]

So you want the commit history to say "r=Manishearth" if I delegate=Foo on a PR? Delegate isn't just used for this purpose, though (on Rust we're using it to train people)

I instruct folks to @bors r=Manishearth if I delegate.

Perhaps only delegate+ (not delegate=) followed by r+ should make the delegator appear as the reviewer?

[jdm]

Oh yeah, I forgot about the granting of power to non-reviewer folk. delegate+ as a heuristic makes sense to me.