Simplify cache API

Simplify cache API

Earlier, add_file_to_index() invalidated the path in the cache-tree
but remove_file_from_cache() did not, and the user of the latter
needed to invalidate the entry himself.  This led to a few bugs due to
missed invalidate calls already.  This patch makes the management of
cache-tree less error prone by making more invalidate calls from lower
level cache API functions.

The rules are:

 - If you are going to write the index, you should either maintain
   cache_tree correctly.

   - If you cannot, alternatively you can remove the entire cache_tree
     by calling cache_tree_free() before you call write_cache().

   - When you modify the index, cache_tree_invalidate_path() should be
     called with the path you are modifying, to discard the entry from
     the cache-tree structure.

 - The following cache API functions exported from read-cache.c (and
   the macro whose names have "cache" instead of "index")
   automatically call cache_tree_invalidate_path() for you:

   - remove_file_from_index();
   - add_file_to_index();
   - add_index_entry();

   You can modify the index bypassing the above API functions
   (e.g. find an existing cache entry from the index and modify it in
   place).  You need to call cache_tree_invalidate_path() yourself in
   such a case.

Signed-off-by: Junio C Hamano <gitster@pobox.com>