So here’s a weird thought: can you merge two repositories in JJ? I have just ended up in a weird state (for unimportant reasons) where I had a remote repo and a local repo that diverged entirely due to me rewriting the commit history. I was going to delete the local copy and re-clone (since the remote was now the source of truth) but I thought “what happens if I add the remote and fetch from it?”, and thought I might as well satisfy my curiosity first.

I added the remote and fetched it, and sure enough it worked without any issues. Since there were no commits in common, I was left with two diverging chains starting at the root commit (an empty commit present at the base of all JJ repos). I could then delete the unwanted chain of commits and get to the state I’d be in if I’d just done a fresh clone.

Or I could do one jj rebase and move all the commits in one chain atop the commits in the other, effectively rebasing one repo on another. This led me to the cursed realisation that I could take two completely unrelated repos and just splice them together, resulting in a repo with the contents and commit histories of both. Of course this will almost certainly result in horrible merge conflicts, but you can do it.