Fix reconciler not working after reapply when was still queued

Created by: eseliger

This fixes an error where the reconciler would determine an empty delta between changeset specs, when the user applies again before a changeset record was processed or had errored.

I hope I am not missing something. @mrnugget asking you for special care when reviewing as you probably have the most insight into the rewirer/reconciler since you originally wrote most of them. I hope this doesn't have other side-effects.

Closes https://github.com/sourcegraph/sourcegraph/issues/16041