copies: rearrange all value comparison conditional 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 changesets. Differential Revision: https://phab.mercurial-scm.org/D9612