Enable retry choose_reviewer

[h-michael]

Sometimes, highfive fails assign reviewer. ex(rust-lang/rust#57913)
If PR owner doesn't know appropriate reviewer, they can't write r? by themselves.
So I want the way that re choose_reviewer action.
Something like r? _retry, r? _.(We can't use underscore to Github username)

[emilyalbini]

Thanks for reporting this! I'd prefer if we fix the underlying cause of highfive crashing instead of adding patches to retry failures though. That specific crash was caused by #190.

[dwijnand]

I think the better workaround for #190 is #187.

[SimonSapin]

I’ve opened #241 for a different use case, but the requested functionality is the same as here.