Fix "git checkout --recurse-submodules" of a nested submodule
hierarchy.
* pb/recurse-submodules-fix:
t/lib-submodule-update: add test removing nested submodules
unpack-trees: check for missing submodule directory in merged_entry
unpack-trees: remove outdated description for verify_clean_submodule
t/lib-submodule-update: move a test to the right section
t/lib-submodule-update: remove outdated test description
t7112: remove mention of KNOWN_FAILURE_SUBMODULE_RECURSIVE_NESTED