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