When trying to fix up a corrupt repository, one might prefer that
"update-index -h" print an accurate usage message and exit rather
than reading the repository and complaining about the corruption.
[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>