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 into the
recursion when the '--recursive' option is given. But this isn't necessary
because all the variables set by the options will be set in the recursion
too because that is done via "eval cmd_update".

Somthing similar was done for cmd_status() in e15bec0ec, so let's clean up
cmd_update() too. Add a test to make sure that a submodule name given on
the command line is not passed into the recursion (which was the intention
of adding the orig_flags variable in 98dbe63db, but is easier achieved by
just dropping it).

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