Update write-tree to use cache-tree.

Update write-tree to use cache-tree.

The updated write-tree reads from $GIT_DIR/index.aux to pick up
subtree objects information, updates the cache-tree with the
index, and updates index.aux file after writing a tree out of
the index file.

Until update-index and other programs that modify the index are
updated to maintain index.aux file, the index.aux file written
by the last write-tree will become stale immediately after they
update the index, which will result in the whole tree
recomputation just like the original write-tree.

The idea is to convert those commands to invalidate cache-tree
whenever they touch the index entries, and write updated
index.aux out.  After the index is updated with them, write-tree
will be able to reuse the parts of the cache-tree that have not
been touched.

Signed-off-by: Junio C Hamano <junkio@cox.net>