"git submodule update" is getting rewritten piece-by-piece into C.

* sb/submodule-update-in-c:
  submodule--helper: introduce new update-module-mode helper
  submodule--helper: replace connect-gitdir-workingtree by ensure-core-worktree
  builtin/submodule--helper: factor out method to update a single submodule
  builtin/submodule--helper: store update_clone information in a struct
  builtin/submodule--helper: factor out submodule updating
  git-submodule.sh: rename unused variables
  git-submodule.sh: align error reporting for update mode to use path