Note for future self on how to successfully cleanly copy an entire git repository from GitHub to SourceHut, with all branches and tags intact.
This is how I did it for wp-cron-pixie:
# on sourcehut, create new repo, then ...
git clone --bare git@github.com:ianmjones/wp-cron-pixie.git
cd wp-cron-pixie.git/
git branch -m master trunk # optional, rename default branch
git push --mirror git@git.sr.ht:~ianmjones/wp-cron-pixie
# on sourcehut, update default branch in repo's settings if not correct.
cd ../
rm -rf wp-cron-pixie.git
git clone git@git.sr.ht:~ianmjones/wp-cron-pixie
cd wp-cron-pixie/
# Check everything is as expected ...
git branch -a
git tag -l
# on github, archive old repo.