submodule--helper: don't call submodule_strategy_to_string() in BUG()

submodule--helper: don't call submodule_strategy_to_string() in BUG()

Don't call submodule_strategy_to_string() in a BUG() message. These
calls added in c51f8f94e5b (submodule--helper: run update procedures
from C, 2021-08-24) don't need the extra information
submodule_strategy_to_string() gives us, as we'll never reach the
SM_UPDATE_COMMAND case here.

That case is the only one where we'd get any information beyond the
straightforward number-to-string mapping.

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>