Group query results under the "ANCESTOR" operator unconditionally

Summary:
Fixes T12753. See that task for reproduction instructions.

We add a `GROUP BY` clause to queries with an "ANCESTOR" edge constraint only if the constaint has more than one PHID, but this is incorrect: the same row can be found twice by an ANCESTOR query if task T is tagged with both "B" and "C", children of "A", and the user queries for "tasks in A".

Instead, always add GROUP BY for ANCESTOR queries.

Test Plan:
  - Followed test plan in T12753.
  - Saw proper paging controls after change.
  - Saw `GROUP BY` in DarkConsole.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12753

Differential Revision: https://secure.phabricator.com/D18012

src/infrastructure/query/policy/PhabricatorCursorPagedPolicyAwareQuery.php

@@ -1811,11 +1811,16 @@ abstract class PhabricatorCursorPagedPolicyAwareQuery
           case PhabricatorQueryConstraint::OPERATOR_NOT:
           case PhabricatorQueryConstraint::OPERATOR_AND:
           case PhabricatorQueryConstraint::OPERATOR_OR:
-          case PhabricatorQueryConstraint::OPERATOR_ANCESTOR:
             if (count($list) > 1) {
               return true;
             }
             break;
+          case PhabricatorQueryConstraint::OPERATOR_ANCESTOR:
+            // NOTE: We must always group query results rows when using an
+            // "ANCESTOR" operator because a single task may be related to
+            // two different descendants of a particular ancestor. For
+            // discussion, see T12753.
+            return true;
           case PhabricatorQueryConstraint::OPERATOR_NULL:
             return true;
         }