Tighten the API to make it harder to misuse in-tree .gitmodules
file, even though it shares the same syntax with configuration
files, to read random configuration items from it.

* ao/config-from-gitmodules:
  submodule-config: reuse config_from_gitmodules in repo_read_gitmodules
  submodule-config: pass repository as argument to config_from_gitmodules
  submodule-config: make 'config_from_gitmodules' private
  submodule-config: add helper to get 'update-clone' config from .gitmodules
  submodule-config: add helper function to get 'fetch' config from .gitmodules
  config: move config_from_gitmodules to submodule-config.c