ls-files -h: show usage even with corrupt index

ls-files -h: show usage even with corrupt index

Part of a campaign to avoid git <command> -h being distracted by
access to the repository.  A caller hoping to use "git ls-files"
with an alternate index as part of a repair operation may well use
"git ls-files -h" to show usage while planning it out.

[jn: with rewritten log message and 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>