* 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()