Multi-ancestor read-tree notes

I've got a version of read-tree which accepts multiple ancestors and does a merge using information from all of them.

The basic features are that it looks for an ancestor which would permit a trivial merge, and uses that. However, if it finds ancestors which permit different trivial merges, it does not merge (which I call case #16).

In case #16, I'm not sure what I should produce. I think the best thing might be to not leave anything in stage 1. The desired end effect is that the user is given a file with a section like:

  {
    *t = NULL;
    *m = 0;
<<<<<<<<
    return Z_DATA_ERROR;
========
    return Z_OK;

  }

In other news, the merge that was giving Len Brown problems a while ago turns out to have the above conflict, and he happened to end up doing the right thing and not reverting Linus's revert of an unnecessary (but harmless) change. I only noticed this just now, when I was testing that merge, and got it to generate only two conflicts regardless of order of ancestors (didn't try to resolve the other one, drivers/acpi/osl.c, with "merge" either way).

So this test is encouraging: I get fewer non-trivial cases than either of the ancestors alone gives, and I catch a case that both single ancestors gets wrong.

Note that there are still some memory leaks for me to fix, but that's the only flaw I know of with this.

Patches against mainline to follow shortly.

	-Daniel
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