lib-submodule-update: consolidate --recurse-submodules

lib-submodule-update: consolidate --recurse-submodules

Both test_submodule_switch_recursing_with_args() and
test_submodule_forced_switch_recursing_with_args() call the internal
function test_submodule_recursing_with_args_common() with the final
argument of `--recurse-submodules`. Consolidate this duplication by
appending the argument in test_submodule_recursing_with_args_common().

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>