Still working on [[todo/import_tree]] today. I decided to
make the imported commit be on an unrelated history from the main branch,
which avoids some very susprising merge behavior, but means
users will need to pass --allow-unrelated-histories to git merge.

Also got `export` and `sync` commands updating the remote tracking branch.
It was surprisingly complicated to do.

With that done, I've tested exporting to a directory remote,
then making changes to the directory manually, and importing, and it all
works together.
