[PATCH 0/3] Handling merge conflicts a bit more gracefully

This series consists of three patches.

  [PATCH 1/3] read-tree.c: rename local variables used in 3-way merge code.
  [PATCH 2/3] read-tree -m 3-way: loosen index requirements that is too strict.
  [PATCH 3/3] read-tree -m 3-way: handle more trivial merges internally

You may have noticed that I already described some "alternative semantics" in the 3-way merge test script t1000. This set of patches implements some of them, namely the following 5 cases:

     O       A       B         result      index requirements
-------------------------------------------------------------------
  5  missing exists  A==B      take A      must match A, if exists.
 ------------------------------------------------------------------
  6  exists  missing missing   remove      must not exist.
 ------------------------------------------------------------------
  8  exists  missing O==B      remove      must not exist.
 ------------------------------------------------------------------
 10  exists  O==A    missing   remove      must match A and be
                                           up-to-date, if exists.
 ------------------------------------------------------------------
 14  exists  O==A    O!=B      take B      if exists, must either (1)
                                           match A and be up-to-date,
                                           or (2) match B.
-------------------------------------------------------------------

The first patch is to match the local variable names used in the functions involved to the names used in the case matrix.

Case #14 is resolved identically as the old code does, but the index requirement old code placed on this case was stricter than necessary. In this case, satisfying the usual rule of "match A and be up-to-date if exists" is certainly OK, but additionally, if the original index matches the tree being merged (without even being up-to-date) is also permissible, because there would be no information loss or work-tree clobbering if we allowed it. The second patch in the series corrects this.

Case #5, #6, #8, and #10 were traditionally kept unmerged in the index file when read-tree is done, and resolving them was left to the script. By resolving these internally, we can loosen the index requirements without compromising correctness for case #5. Other three cases could still be left for the "script policy" because this change does not affect the index requirements for these cases, but it was simple enough to implement them and this would not be too controversial a change. The third patch in the series implements these changes.