This separates out the part that deals with one-commit diff-tree
(and --stdin form) into a separate log-tree module.
There are two goals with this. The more important one is to be
able to make this part available to "git log --diff", so that we
can have a native "git whatchanged" command. Another is to
simplify the commit log generation part simpler.
Signed-off-by: Junio C Hamano <junkio@cox.net>
