Correct behavior of "writable" Almanac service binding for repository services

Summary: Ref T13611. This property worked correctly when implemented in D19357. The behavior was broken by D20775, which tested node-level routing but did not specifically re-test the "writable" property. This was difficult to spot because ref query outcomes weren't observable in the UI, and the ref itself had the correct property value.

Test Plan:
See D21575. After this change, the UI shows the correct state, rather than showing a read-only service ref as writable:

{F8465865}

Maniphest Tasks: T13611

Differential Revision: https://secure.phabricator.com/D21576

src/applications/repository/storage/PhabricatorRepository.php

@@ -1975,7 +1975,7 @@ final class PhabricatorRepository extends PhabricatorRepositoryDAO
     if ($writable) {
       foreach ($refs as $key => $ref) {
         if (!$ref->isWritable()) {
-          unset($results[$key]);
+          unset($refs[$key]);
         }
       }