Code clean-up to avoid mixing values read from the .gitmodules file
and values read from the .git/config file.

* bw/submodule-config-cleanup:
  submodule: remove gitmodules_config
  unpack-trees: improve loading of .gitmodules
  submodule-config: lazy-load a repository's .gitmodules file
  submodule-config: move submodule-config functions to submodule-config.c
  submodule-config: remove support for overlaying repository config
  diff: stop allowing diff to have submodules configured in .git/config
  submodule: remove submodule_config callback routine
  unpack-trees: don't respect submodule.update
  submodule: don't rely on overlayed config when setting diffopts
  fetch: don't overlay config with submodule-config
  submodule--helper: don't overlay config in update-clone
  submodule--helper: don't overlay config in remote_submodule_branch
  add, reset: ensure submodules can be added or reset
  submodule: don't use submodule_from_name
  t7411: check configuration parsing errors