Set correct Drupal root when a relative path is provided

[weitzman]

The Finder does a chdir when --root is provided. This complicates later processing of --root in drush_preflight_root() when a relative path is provided (e.g. -r web). That relative path is not transformed into an absolute path by the shell, at least not on Windows.

[weitzman]

Perhaps we need to unset the -r/--root option if we handle it in the Finder?