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