Merge branch 'ar/submodule-add'

Rewrite of "git submodule" in C continues. * ar/submodule-add: submodule: drop unused sm_name parameter from show_fetch_remotes() 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