Clean up virtual "_ft_rank" column for query construction of Ferret objects

Summary:
Ref T12974. Ferret object queries SELECT a virtual "_ft_rank" column for relevance ordering.

Currently, they always SELECT this column. That's fine and doesn't hurt anything, but makes developing and debugging things kind of a pain since every query has this `, blah blah _ft_rank` junk.

Instead, construct this column only if we're actually going to use it.

Mostly, this cleans up DarkConsole / query logs a bit.

Test Plan:
Viewed normal query results on various pages, viewed global search results, ordered Maniphest tasks by normal stuff and by "Relevance".

Viewed DarkConsole, saw no more "_ft_rank" junk on normal pages.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12974

Differential Revision: https://secure.phabricator.com/D18728

src/infrastructure/query/policy/PhabricatorCursorPagedPolicyAwareQuery.php

@@ -1565,6 +1565,13 @@ abstract class PhabricatorCursorPagedPolicyAwareQuery
       return $select;
     }
 
+    $vector = $this->getOrderVector();
+    if (!$vector->containsKey('rank')) {
+      // We only need to SELECT the virtual "_ft_rank" column if we're
+      // actually sorting the results by rank.
+      return $select;
+    }
+
     if (!$this->ferretEngine) {
       $select[] = '0 _ft_rank';
       return $select;