<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: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>