check-ref-format --branch requires access to the repository
to resolve refs like @{-1}.
Noticed by Nguyễn Thái Ngọc Duy.
Cc: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>