sync might not work with worktree

I get conflicts when syncing after a merge `gh stack sync` has a warning

```
⚠ Failed to fast-forward master: failed to run git: fatal: cannot force update the branch 'master' used by worktree at 'XXX'
```

seems the fix is to `git rebase --onto origin/master` but `gh stack rebase` does not have `--onto`