Merge branch 'cs/add-skip-submodule-ignore-all' into seen

Merge branch 'cs/add-skip-submodule-ignore-all' into seen

"git add <submodule>" has been taught to honor
submodule.<name>.ignore that is set to "all" (and requires "git add
-f" to override it).

* cs/add-skip-submodule-ignore-all:
  Documentation: update add --force option + ignore=all config
  tests: fix existing tests when add an ignore=all submodule
  tests: t2206-add-submodule-ignored: ignore=all and add --force tests
  read-cache: submodule add need --force given ignore=all configuration
  read-cache: update add_files_to_cache take param ignored_too