builtin/submodule--helper.c: rename option struct to "opt"

builtin/submodule--helper.c: rename option struct to "opt"

In a later commit, update_clone()'s options will be stored in a struct
update_data instead of submodule_update_clone. Preemptively rename the
options struct to "opt" to shrink that commit's diff.

Helped-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Glen Choo <redacted>
Signed-off-by: Junio C Hamano <redacted>