Ci-fairy without FDO_UPSTREAM_REPO

I ran into an issue where FDO_UPSTREAM_REPO was not set, and ci-fairy as a result silently failing.

I see that ci-fairy tries to handle this by using the repo of the source branch to replace the upstream repo. If we instead first try the repo path of the merge request repo, it should find the right one without having to have FDO_UPSTREAM_REPO set. Is there a reason for this not being done?