Merge branch 'bw/submodule-sans-cache-compat'

Code clean-up.

* bw/submodule-sans-cache-compat:
submodule: convert get_next_submodule to not rely on the_index
submodule: used correct index in is_staging_gitmodules_ok
submodule: convert stage_updated_gitmodules to take a struct index_state

diff --cc submodule.c
Simple merge