3-way read-tree case matrix.

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"LT" == Linus Torvalds [off-list ref] writes:

LT> I did. It looks fine, although I'd like to point out that even the
LT> three-way merge _does_ mix in the original state for the case where the
LT> merge was trivially resolved...

Thanks.  I have another request for sanity checking.

I am not sure if salvaging a dirty tree in a 3-way merge (as
opposed to just downright refusing) would have as much practical
value as the 2-tree fast-forward case, but here is a proposed
case matrix for the 3-way merge.

"git-diff-tree -m O H M"

    O       H       M         result      index requirements
------------------------------------------------------------------
  1 missing missing missing   -           must not exist.
 -----------------------------------------------------------------
  2 missing missing exists    take M      must match M if exists.
 -----------------------------------------------------------------
  3 missing exists  missing   remove      must match H and be
					  up-to-date, if exists.
 -----------------------------------------------------------------
  4 missing exists  exists    no merge    must match H and be
					  up-to-date, if exists.
 -----------------------------------------------------------------
  5 exists  missing missing   no merge    must not exist.
 -----------------------------------------------------------------
  6 exists  missing O==M      remove      must not exist.
 -----------------------------------------------------------------
  7 exists  missing O!=M      no merge    must not exist.
 -----------------------------------------------------------------
  8 exists  O==H    missing   remove      must match H and be
					  up-to-date, if exists.
 -----------------------------------------------------------------
  9 exists  O!=H    missing   no merge    must match H and be
					  up-to-date, if exists.
 -----------------------------------------------------------------
 10 exists  O!=H    O!=M      no merge    must match H and be
					  up-to-date, if exists.
 -----------------------------------------------------------------
 11 exists  O!=H    O==M      take H      must match H, if exists.
 -----------------------------------------------------------------
 12 exists  O==H    O!=M      take M      if exists, must either (1)
    					  match H and be up-to-date,
                                          or (2) match M.
 -----------------------------------------------------------------
 13 exists  O==H    O==M      take M      must match H if exists.
------------------------------------------------------------------

In all "take H" or "take M" cases, if the original index matches
what is taken, I would reuse it, and keep it dirty if it is.

The goal is, "(1) do not clobber the current index; (2) arrive
at the same result as in the case started with an empty index;
(3) favor success over failure as long as (1) and (2) are
satisfied".