Merge branch 'jc/checkout-B-branch-in-use'

"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