Rebasing from a worktree fails if main is checked out

[rogueg]

While on a worktree, I got

> gh stack rebase
✓ Fetched origin
⚠ Failed to fast-forward main: failed to run git: fatal: cannot force update the branch 'main' used by worktree at '/co/main'

Stack detected: (main) <- tf/prod <- tf/subnets
Rebasing branches in order, starting from tf/prod-name to tf/subnets
✓ Skipping tf/prod (PR #99 merged)
⚠ Rebasing tf/subnets onto main — conflict

I was able to fix it by detaching from the main branch in the main worktree. There were no true conflicts in this stack.

Seems to me there are two issues:

1. stacks should run against origin/main, rather than local main.
2. failing to update main should abort immediately, rather than attempt the rebase.