Merge branch 'ar/submodule-add-config'

Large part of "git submodule add" gets rewritten in C.

* ar/submodule-add-config:
  submodule--helper: introduce add-config subcommand

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc git-submodule.sh
Simple merge

diff --cc submodule.c
Simple merge