Merge branch 'ja/worktree-orphan'

'git worktree add' learned how to create a worktree based on an orphaned branch with `--orphan`. * ja/worktree-orphan: worktree add: emit warn when there is a bad HEAD worktree add: extend DWIM to infer --orphan worktree add: introduce "try --orphan" hint worktree add: add --orphan flag t2400: add tests to verify --quiet t2400: refactor "worktree add" opt exclusion tests t2400: cleanup created worktree in test worktree add: include -B in usage docs