t7406: Fix submodule init config tests

t7406: Fix submodule init config tests

These tests have been broken since they were introduced in commits
ca2cedb (git-submodule: add support for --rebase., 2009-04-24) and
42b4917 (git-submodule: add support for --merge., 2009-06-03).
'git submodule init' expects the submodules to exist in the index.
In this case, the submodules don't exist and therefore looking for
the submodules will always fail. To make matters worse, git submodule
fails visibly to the user by saying:

error: pathspec 'rebasing' did not match any file(s) known to git.
Did you forget to 'git add'?

but doesn't return an error code. This allows the test to fail silently.
Fix it by adding the submodules first.

Cc: Johan Herland <johan@herland.net>
Cc: Peter Hutterer <peter.hutterer@who-t.net>
Cc: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>