Merge branch 'bw/grep-recurse-submodules' into next

Merge branch 'bw/grep-recurse-submodules' into next

"git grep --recurse-submodules" has been reworked to give a more
consistent output across submodule boundary (and do its thing
without having to fork a separate process).

* bw/grep-recurse-submodules:
  grep: recurse in-process using 'struct repository'
  submodule: merge repo_read_gitmodules and gitmodules_config
  submodule: check for unmerged .gitmodules outside of config parsing
  submodule: check for unstaged .gitmodules outside of config parsing
  submodule: remove fetch.recursesubmodules from submodule-config parsing
  submodule: remove submodule.fetchjobs from submodule-config parsing
  config: add config_from_gitmodules
  cache.h: add GITMODULES_FILE macro
  repository: have the_repository use the_index
  repo_read_index: don't discard the index