in-core index API ================= Talk about and , things like: * cache -> the_index macros * read_index() * write_index() * ie_match_stat() and ie_modified(); how they are different and when to use which. * index_name_pos() * remove_index_entry_at() * remove_file_from_index() * add_file_to_index() * add_index_entry() * refresh_index() * discard_index() * cache_tree_invalidate_path() * cache_tree_update() (JC, Linus)