RFH: refactor read-tree

Hi,

the last thing to do with merge-recursive to speed it up, would be to 
avoid reading/writing the cache all the time.

Unfortunately, builtin-read-tree.c grew into a pretty big monster, with so 
many different options which completely change behaviour.

So, how should I go about it? Should I make a struct a la diff_options to 
hold the options to unpack_trees? Where should it go?

I also played a little with git-merge-tree, because it seems so much 
simpler and easier to refactor. But there is a problem: Either I call it 
the wrong way, or it does not yet work correctly: I tried

	git-merge-tree $(git-merge-base branch1 branch2) branch1 branch2

with what is in 'next'. But it only showed the _new_ files, not the 
modified ones.

Help, please?

Ciao,
Dscho