Show usage string for 'git check-ref-format -h'

Show usage string for 'git check-ref-format -h'

This only changes the behavior of "git check-ref-format -h"
without any other options and arguments.

This change cannot be breaking backward compatibility, since any
valid refname must contain a /.   Most existing scripts use
arguments such as "heads/$foo".  If some script checks the
refname "-h" alone, git check-ref-format will still exit with
nonzero status, and the only detrimental side-effect will be a
usage string sent to stderr.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>