Campaigns show "Revision not found" error when merged changeset's branch is deleted upstream

Created by: mrnugget

A customer reports:

One thing I noticed now is this kind of error message appears. We usually delete the branches of a pull-request on merge and it seems like this makes sourcegraph lose track if the state

The error mesage is:

Revision not found: <remote-bitbucket-server-url>/PROJ/proj@refs/heads/<branch-name>