Merge branch 'ar/submodule-add' into next

Rewrite of "git submodule" in C continues.

* ar/submodule-add:
  submodule--helper: introduce add-clone subcommand
  submodule--helper: refactor module_clone()
  submodule: prefix die messages with 'fatal'
  t7400: test failure to add submodule in tracked path

Author: gitster