Merge branch 'gc/submodule-update-part2'

Move more "git submodule update" to C. * gc/submodule-update-part2: submodule--helper: remove forward declaration submodule: move core cmd_update() logic to C submodule--helper: reduce logic in run_update_procedure() submodule--helper: teach update_data more options builtin/submodule--helper.c: rename option struct to "opt" submodule update: use die_message() submodule--helper: run update using child process struct