Merge branch 'sb/checkout-recurse-submodules'

Merge branch 'sb/checkout-recurse-submodules'

"git checkout" is taught the "--recurse-submodules" option.

* sb/checkout-recurse-submodules:
  builtin/read-tree: add --recurse-submodules switch
  builtin/checkout: add --recurse-submodules switch
  entry.c: create submodules when interesting
  unpack-trees: check if we can perform the operation for submodules
  unpack-trees: pass old oid to verify_clean_submodule
  update submodules: add submodule_move_head
  submodule.c: get_super_prefix_or_empty
  update submodules: move up prepare_submodule_repo_env
  submodules: introduce check to see whether to touch a submodule
  update submodules: add a config option to determine if submodules are updated
  update submodules: add submodule config parsing
  make is_submodule_populated gently
  lib-submodule-update.sh: define tests for recursing into submodules
  lib-submodule-update.sh: replace sha1 by hash
  lib-submodule-update: teach test_submodule_content the -C <dir> flag
  lib-submodule-update.sh: do not use ./. as submodule remote
  lib-submodule-update.sh: reorder create_lib_submodule_repo
  submodule--helper.c: remove duplicate code
  connect_work_tree_and_git_dir: safely create leading directories