contrib/packaging/wix/defines.wxi

extdiff: remove dir2root and pass full path as dir2 in _runperfilediff()

The only use of `dir2root` was to join with `dir2` to generate the path for
other side of diff. Like in previous patch, `dir1a` and `dir1b` are full paths
and no longer base names, hence we pass `dir2` as full path too and making
`dir2root` unrequired.

Differential Revision: https://phab.mercurial-scm.org/D8970

<Include>

  <?if $(var.Platform) = "x64" ?>
    <?define IsX64 = yes ?>
  <?else?>
    <?define IsX64 = no ?>
  <?endif?>

</Include>