Should not be able to invite reviewers who can't currently see branch

[Michael Nelson]

I think there are two issues really:

(1) It's too easy to add the wrong reviewer to a branch. I made the mistake of adding 'abel' as a reviewer of my branch, but it was not the abel I was expecting.

(2) There is no way to remove a person as a reviewer (to effectively revoke their access to the branch and the branches ancestors - if that's relevant).

Possible options for (1):
a) Limiting the possible reviewers to only those people who have access to the branch (important for private branches).

b) Perhaps a confirmation: "You are about to add <users full name and email if available?> as a reviewer of this branch."? Not sure, maybe that's overkill, but helps confirm that the entered userid corresponds to the real person intended.