When merged changesets are detached/archived they are set to be closed

Created by: mrnugget

This code here does not check whether the changeset is already closed/merged before marking as "Closing":

https://github.com/sourcegraph/sourcegraph/blob/2ed5672306e62e0b4314c662568df1098cc5babf/enterprise/internal/batches/rewirer/rewirer.go#L163-L195

That leads to merged changesets showing up as to be closed: