Show usage string for 'git unpack-file -h'

"unpack-file -h" could be asking to save the contents of a blob
named "-h".  Strictly speaking, such a pathological ref name is
possible, but the user would have to had said something like
"tags/-h" to name such a pathological ref already.  When used in
scripts, unpack-file is typically not passed a user-supplied tag
name directly.

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

unpack-file.c

@@ -28,7 +28,7 @@ int main(int argc, char **argv)
 
 	git_extract_argv0_path(argv[0]);
 
-	if (argc != 2)
+	if (argc != 2 || !strcmp(argv[1], "-h"))
 		usage("git unpack-file <sha1>");
 	if (get_sha1(argv[1], sha1))
 		die("Not a valid object name %s", argv[1]);