parse-options: Allow to hide options from the default usage. This is useful for backward-compatibility aliases, or very advanced command line switches introduced for internal git usages and have no real use for a user. parse-options still shows them if the user asks for --help-all. Signed-off-by: Pierre Habouzit <madcoder@debian.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>