Function force_update_branch 

pub fn force_update_branch(
    branch: &str,
    target: &str,
    verbose: bool,
) -> Result<()>

Force-move a local branch ref to target (git branch -f).

Git itself refuses when branch is checked out in ANY worktree, so this can never yank a working tree out from under anyone; callers use it to fast-forward a branch that is not checked out (see helpers::fast_forward_home_ref).