You need to make sure both branches are using a rich-root-format. You can do this by upgrading both to the rich-root-pack format, e.g. "bzr upgrade --rich-root-pack bzr+ssh://<email address hidden>/%7Ebbs2ch-bot/bbs2ch/0.4-mirror", "bzr upgrade --rich-root-pack .".
The error message from bzr could perhaps be a bit clearer.
You need to make sure both branches are using a rich-root-format. You can do this by upgrading both to the rich-root-pack format, e.g. "bzr upgrade --rich-root-pack bzr+ssh://<email address hidden> /%7Ebbs2ch- bot/bbs2ch/ 0.4-mirror" , "bzr upgrade --rich-root-pack .".
The error message from bzr could perhaps be a bit clearer.