Merge branch 'ar/submodule-add-more'

Merge branch 'ar/submodule-add-more'

More parts of "git submodule add" has been rewritten in C.

* ar/submodule-add-more:
  submodule--helper: rename compute_submodule_clone_url()
  submodule--helper: remove resolve-relative-url subcommand
  submodule--helper: remove add-config subcommand
  submodule--helper: remove add-clone subcommand
  submodule--helper: convert the bulk of cmd_add() to C
  dir: libify and export helper functions from clone.c
  submodule--helper: remove repeated code in sync_submodule()
  submodule--helper: refactor resolve_relative_url() helper
  submodule--helper: add options for compute_submodule_clone_url()