Merge branch 'sb/checkout-recurse-submodules' into maint

"git checkout --recurse-submodules" did not quite work with a
submodule that itself has submodules.

* sb/checkout-recurse-submodules:
  submodule: properly recurse for read-tree and checkout
  submodule: avoid auto-discovery in new working tree manipulator code
  submodule_move_head: reuse child_process structure for futher commands