Merge branch 'ja/worktree-orphan'

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