Merging APT overlays fails: No distribution definition of '[…]' found in './conf/distributions'!

During the 4.12 release process, this step fails:

$ for overlay in $(ls config/APT_overlays.d/) ; do
           if ! ssh reprepro@incoming.deb.tails.boum.org \
                   tails-merge-suite "$overlay" "$BRANCH" ; then
              echo "Failed to merge '$overlay' into '$BRANCH': $?" >&2
              break
           fi
done
Already on 'stable'
Your branch is ahead of 'origin/stable' by 40 commits.
  (use "git push" to publish your local commits)
No distribution definition of 'bugfix-17962-reintroduce-thunderbird-68' found in './conf/distributions'!
There have been errors!
No distribution definition of 'feature-17896-linux-5.8-force-all-tests' found in './conf/distributions'!
There have been errors!

That's because GitLab MRs have "delete source branches" enabled by default. So when merging the MRs that introduced these 2 overlays, the Git branches were deleted. And in turn, the corresponding APT overlay suites were removed from the reprepro configuration (although the exported suite is still present and usable by APT: we never delete such data).