in-core index API
=================

Talk about <read-cache.c> and <cache-tree.c>, 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)