submodule update: remove unnecessary orig_flags variable

cmd_update() in the submodule script tries to preserve the options given
on the command line in the "orig_flags" variable to pass them on into the
recursion when the '--recursive' option is given. But this isn't necessary
because all the variables set by the options will be seen in the recursion
too as that is achieved by executing "eval cmd_update".

The same has already been done for cmd_status() in e15bec0ec, so let's
clean up cmd_update() likewise. Also add a test to make sure that a
submodule name given on the command line is not passed into the recursion
(which was the goal of adding the orig_flags variable in 98dbe63db).

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>