r? @rust-highfive to reassign a review randomly (again)

[SimonSapin]

When a PR is first opened, highfive randomly assigns its review to someone. Based on https://github.com/rust-lang/highfive/blob/master/highfive/configs/rust-lang/rust.json#L10 it looks like this is based on what files/directories the PR modifies. This is all good.

Sometimes, for a variety of possible reasons, the review an already-open PR needs to be reassigned to someone else. Typically what happens is that the person doing this navigates the appropriate team’s page under https://www.rust-lang.org/governance/ and picks someone at semi-random. It would be nice to have a way to ask highfive to auto-assign again, like it did when the PR was first opened:

• This is fewer steps for the person who decided that reassigning was needed
• In some cases we have deliberate differences between team membership and auto-assigned reviewers

Ideally highfive would in that case pick someone different from the first time on that particular PR. However running the same algorithm again would be good already: there are usually enough reviewers to choose from that picking the same twice in a row is unlikely, and if it happens we can simply give the command again.

The exact command to trigger this is less important. I initially though of r? @rust-highfive (we "assign" to highfive the responsibility to reassign someone else), #189 proposed r? _retry or r? _

[joshtriplett]

This has come up many more times since it was reported. It'd be really helpful to have this kind of "random reassign" mechanism, for several different teams.