Existing merged changesets are “closed” and hence removed from the tracking campaign

Created by: eseliger

• Sourcegraph version: master
• Platform information: Dogfood

Steps to reproduce:

1. Have a campaign with some changesets already merged
2. Run src-cli again to create the newest patches (I wanted to expand the scope)
3. See them marked as "to be closed" (pr for a "unchanged" tab is up)
4. Click update, see them disappear

Expected behavior:

The 4 merged changesets would have been skipped, retaining them in the campaign.

Actual behavior:

The 4 merged changesets were interpreted as "to be closed" and are removed from the campaign. Lost tracking progress of them.