submodule--helper style: don't separate declared variables with \n\n

submodule--helper style: don't separate declared variables with \n\n

The usual style in the codebase is to separate declared variables with
a single newline, not two, let's adjust this code to conform to
that. This makes the eventual addition of various "int ret" variables
more consistent.

In doing this the comment added in 2964d6e5e1e (submodule: port
subcommand 'set-branch' from shell to C, 2020-06-02) might become
ambiguous to some, although it should be clear what it's referring to,
let's move it above the 'OPT_NOOP_NOARG('q', "quiet")' to make that
clearer.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>