Merge branch 'ab/cache-api-cleanup'

Code clean-up to tighten the use of in-core index in the API. * ab/cache-api-cleanup: cache API: add a "INDEX_STATE_INIT" macro/function, add release_index() read-cache.c: refactor set_new_index_sparsity() for subsequent commit sparse-index API: BUG() out on NULL ensure_full_index() sparse-index.c: expand_to_path() can assume non-NULL "istate" builtin/difftool.c: { 0 }-initialize rather than using memset()