scripts: Add placeholders for OPTIONS_SPEC

--text follows this line--
These commands currently lack OPTIONS_SPEC; allow people to
easily list with "git grep 'OPTIONS_SPEC=$'" what they can help
improving.

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

git-submodule.sh

@@ -5,6 +5,7 @@
 # Copyright (c) 2007 Lars Hjemli
 
 USAGE='[--quiet] [--cached] [add <repo> [-b branch]|status|init|update] [--] [<path>...]'
+OPTIONS_SPEC=
 . git-sh-setup
 require_work_tree