move: Ensure that moved branches behave sensibly across worktrees

[arxanas]

[1] By the way, @arxanas, since git move can move multiple branches, I suppose that means you may want to check that the branches you're about to move are not checked out in other working copies (I haven't checked if you already do, but it seems like something that's easy to overlook).

Originally posted by @martinvonz in jj-vcs/jj#13 (comment)

[arxanas]

• If we move a branch, then we should probably detach HEAD in all other worktrees, at the least?
• If you have a rewritten commit checked out, then I suppose git restack ought to move you to the newest version of the commit as remediation.