To properly handle the newly tested case (chaining of merges) we will need to
detect more accurately when an actualy merging of the copy information (and
superseed the two existing data). Before starting to do so, we need to
reorganise the values comparison to introduce different conditional branches
when such actual merging is needed/detected.
To avoid mixing too many change in this complicated code, we do the
reorganisation before adding the "overwrite detection" logic in the next