The two move helpers run inside the WithWaitingStatus worker that
withNewBranchNamePrompt dispatches to, but read Model().Files/Submodules
(to decide whether to auto-stash) and Model().Commits (the unpushed
commits to cherry-pick off the base branch) from there, racing the UI
thread's model writes. Compute mustStash — needed by both paths — at the
top, and the unpushed commits in the off-of-main menu item, on the UI
thread, and pass them into the helpers.