<branch> can be omitted in this syntax, and it's actually documented a
few paragraphs down:

  You could omit <branch>, in which case the command degenerates to
  "check out the current branch", which is a glorified no-op with
  rather expensive side-effects to show only the tracking information,
  if exists, for the current branch.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>