log-tree: separate major part of diff-tree.

log-tree: separate major part of diff-tree.

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>