Merge branch 'jl/submodule-tests' into next

Merge branch 'jl/submodule-tests' into next

* jl/submodule-tests:
  revert: add t3513 for submodule updates
  stash: add t3906 for submodule updates
  am: add t4255 for submodule updates
  cherry-pick: add t3512 for submodule updates
  pull: add t5572 for submodule updates
  rebase: add t3426 for submodule updates
  merge: add t7613 for submodule updates
  bisect: add t6041 for submodule updates
  reset: add t7112 for submodule updates
  read-tree: add t1013 for submodule updates
  apply: add t4137 for submodule updates
  checkout: call the new submodule update test framework
  submodules: add the lib-submodule-update.sh test library
  test-lib: add test_dir_is_empty()