Correct the interaction between overheating and offset-based paging

Summary:
Ref T13386. If you issue `differential.query` with a large offset (like 3000), it can overheat regardless of policy filtering and fail with a nonsensical error message.

This is because the overheating limit is based only on the query limit, not on the offset.

For example, querying for "limit = 100" will never examine more than 1,100 rows, so a query with "limit = 100, offset = 3000" will always fail (provided there are at least that many revisions).

Not all numbers work like you might expect them to becuase there's also a 1024-row fetch window, but basically small limits plus big offsets always fail.

Test Plan: Artificially reduced the internal window size from 1024 to 5, then ran `differential.query` with `offset=50` and `limit=3`. Before: overheated with weird error message. After: clean result.

Maniphest Tasks: T13386

Differential Revision: https://secure.phabricator.com/D20728

src/infrastructure/query/policy/PhabricatorPolicyAwareQuery.php

@@ -233,7 +233,10 @@ abstract class PhabricatorPolicyAwareQuery extends PhabricatorOffsetPagedQuery {
     // number of records when the viewer can see few or none of them. See
     // T11773 for some discussion.
     $this->isOverheated = false;
-    $overheat_limit = $limit * 10;
+
+    // See T13386. If we on an old offset-based paging workflow, we need
+    // to base the overheating limit on both the offset and limit.
+    $overheat_limit = $need * 10;
     $total_seen = 0;
 
     do {