AutoForwardBranches runs in the Then callback of the refresh after a
fetch. This is on the UI thread, so the git command for updating the
branches blocks the UI while it runs. This is fine for a single
update-ref, but the next commit adds a git status and a git merge for
each branch that is checked out in another worktree.
Keep reading the branches from the model on the UI thread, and move the
git work to a worker.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>