(stable) Fix an issue where querying for a large number of projects by slug could paginate incorrectly

Summary:
See PHI1809. This query may join the "slug" table, but each project may have multiple slugs, and the query does not "GROUP BY" when this join occurs.

This may lead to partial result sets and unusual paging behavior.

This could likely be caught categorically in `loadAllFromArray()`; I'll adjust this in a followup.

Test Plan:
A minimal reproduction case is something like:

  - Give project P slugs: a, b, c.
  - Give project Q slugs: d.
  - Query for slugs: a, b, c, d; with limit 2.
  - Order the query so P returns first.
  - Expect: P and Q.
  - Actual: P generates 3 raw rows and the final result is just P with no pagination cursor.

Differential Revision: https://secure.phabricator.com/D21399

src/applications/project/query/PhabricatorProjectQuery.php

@@ -656,6 +656,11 @@ final class PhabricatorProjectQuery
     if ($this->memberPHIDs || $this->watcherPHIDs || $this->nameTokens) {
       return true;
     }
+
+    if ($this->slugs) {
+      return true;
+    }
+
     return parent::shouldGroupQueryResultRows();
   }