resolve: use "other" changeset from merge state (issue4163)
We can use the "other" data from the recorded merge state instead of inferring
what the other could be from working copy parent. This will allow resolve to
fulfil its duty even when the second parent have been dropped.
Most direct benefit is fixing a regression in backout.
<Include>
<?if $(var.Platform) = "x64" ?>
<?define IsX64 = yes ?>
<?else?>
<?define IsX64 = no ?>
<?endif?>
</Include>