Re: 3-way read-tree case matrix.

I suspect my mailer dropped your response on the floor when it
polled my ISP at around 21:27 my time.  I found its log
mentioning your address but no message in my mailbox.

Since the last message I sent you, I updated read-tree.c to
match the proposed behaviour, and I found a couple of problems
with it by running the 3-way merge test we already have.  I am
attaching a revised one.  I have not finished a new test suite
that runs on a populated index file yet, which is what I plan to
do next.

There is one thing that the proposed table changes from the
traditional 3-way merge semantics.  I think this is a sensible
change.

 - #2 and #3 (I botched #3 in the earlier one I sent you);
   traditionally we left <O,H,M>=<none,none,exists> and
   <O,H,M>=<none,exists,none> cases to the script policy; we
   could salvage a (potentially dirty) cache entry if we say we
   pick the one created in only one branch.

Earlier I botched <O,H,M>=<some,H!=O,M!=O> case in the table; we
should collapse to H/M if H==M; this is now handled differently
from H!=O,M!=O,H!=M case (case #10) as case #11 in the updated
table.

------------

"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   take H      must match H, 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
		    M!=H		  up-to-date, if exists.
 -----------------------------------------------------------------
 11 exists  O!=H    O!=M      take H	  must match H, if exists.
		    M==H
 -----------------------------------------------------------------
 12 exists  O!=H    O==M      take H      must match H, if exists.
 -----------------------------------------------------------------
 13 exists  O==H    O!=M      take M      if exists, must either (1)
    					  match H and be up-to-date,
                                          or (2) match M.
 -----------------------------------------------------------------
 14 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, "(a) do not clobber the current index; (b) arrive
at the same result as in the case started with an empty index;
(c) favor success over failure as long as (a) and (b) are
satisfied."