"git checkout -B <branch> [<start-point>]" allowed a branch that is
in use in another worktree to be updated and checked out, which
might be a bit unexpected. The rule has been tightened, which is a
breaking change. "--ignore-other-worktrees" option is required to
unbreak you, if you are used to the current behaviour that "-B"
overrides the safety.
* jc/checkout-B-branch-in-use:
checkout: forbid "-B <branch>" from touching a branch used elsewhere
checkout: refactor die_if_checked_out() caller