Merge branch 'gc/submodule-use-super-prefix'

Another step to rewrite more parts of "git submodule" in C.

* gc/submodule-use-super-prefix:
  submodule--helper: remove display path helper
  submodule--helper update: use --super-prefix
  submodule--helper: remove unused SUPPORT_SUPER_PREFIX flags
  submodule--helper: use correct display path helper
  submodule--helper: don't recreate recursive prefix
  submodule--helper update: use display path helper
  submodule--helper tests: add missing "display path" coverage