Show usage string for 'git grep -h'

Show usage string for 'git grep -h'

Clarification: the following description only talks about "git
grep -h" without any other options and arguments.

Such a change cannot be breaking backward compatibility.  "grep
-h" cannot be asking for suppressing filenames, as there is no
match pattern specified.

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