During the commit process, update the cache-tree. Write this updated
cache-tree so that it's ready for subsequent commands.
Add test code which demonstrates that git commit now writes the cache
tree. Make all tests test the entire cache-tree, not just the root
level.
Signed-off-by: David Turner <dturner@twitter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>