ls-remote: a lone "-h" is asking for help

ls-remote: a lone "-h" is asking for help

What should happen if you run this command?

	$ git ls-remote -h

It does not give a short-help for the command. Instead because "-h" is a
synonym for "--heads", it runs "git ls-remote --heads", and because there
is no remote specified on the command line, we run it against the default
"origin" remote, hence end up doing the same as

	$ git ls-remote --heads origin

Fix this counter-intuitive behaviour by special casing a lone "-h" that
does not have anything else on the command line and calling usage().

Signed-off-by: Junio C Hamano <gitster@pobox.com>