add "bors reroll"

[trevyn]

rust-lang/rust#119752 (comment)

@bors reroll

to pick a new reviewer

this has been haunting me

[Mark-Simulacrum]

bors/homu doesn't handle reviewers at all, so moving to triagebot.

[ehuss]

I'm going to reopen since I don't think this is resolved by #1958. This issue is about asking triagebot to find someone else to assign based on the owner map. If I get assigned a PR and I want to reassign it, I don't want to have to go look at the map to figure out who might be a good candidate. You also can't directly emulate the random choice among a set of different groups and individuals.