rustbot should not throw an error when updating a message containing a assignment

[Urgau]

See comment rust-lang/rust#117522 (comment) saying that petrochenkov is already assigned, which is true, but there isn't any assignment expect in the primary message which isn't new.

I can only deduce that because I updated this message it decided to throw an error, but interestingly it doesn't always do it. Maybe it also has something to do with me update the title at the same time?