gc -h: show usage even with broken configuration

gc -h: show usage even with broken configuration

Given a request for command-line usage information rather than some
more substantial action, the only friendly thing to do is to report
the usage information as soon as possible and exit.

Without this change, as "git gc" glances over the repository, it can
be distracted by the desire to report a malformed configuration file.

Noticed while working through reports from Duy's repository access
checker.

[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>