checkout-index -h: show usage even in an invalid repository

checkout-index -h: show usage even in an invalid repository

checkout-index loads the index before parsing options.  Erroring out
is counterproductive at that point if the operator is hunting for a
command to recover useful data from the broken repository.

[jn: new commit message, tests]

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