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