Fix an issue with editing pre-space objects using a form with no visibility controls

Summary:
WMF ran into this after their update. Here's the setup:

  - When you enable Spaces, we leave all existing objects set to `null`, which means "these belong to the default space". This is so we don't have to go update a trillion objects.
  - New objects get set to the default space explicitly (`PHID-SPCE-...`) but older ones stay with `null`.
  - If you edit an older object (like a task) from the time before Spaces, //and// the form doesn't have a Visbility/Spaces control, we would incorrectly poplate the value with `null` when the effective value should be the default space PHID.
  - This caused a "You must choose a space." error in the UI.

Instead, populate the control with the effective value instead of the literal database value. This makes the edit go through cleanly.

Also add a note about this for future-me.

Test Plan:
  - Disabled "Visibility" control in task edit form.
  - Edited an old task which had `null` as a `spacePHID` in the database.
  - Before patch: UI error about selecting a Space.
  - After patch: edit goes through cleanly.

Reviewers: chad, 20after4

Reviewed By: chad, 20after4

Subscribers: 20after4, aklapper

Differential Revision: https://secure.phabricator.com/D15306

src/applications/policy/editor/PhabricatorPolicyEditEngineExtension.php

@@ -101,6 +101,9 @@ final class PhabricatorPolicyEditEngineExtension
         if ($capability == PhabricatorPolicyCapability::CAN_VIEW) {
           $type_space = PhabricatorTransactions::TYPE_SPACE;
           if (isset($types[$type_space])) {
+            $space_phid = PhabricatorSpacesNamespaceQuery::getObjectSpacePHID(
+              $object);
+
             $space_field = id(new PhabricatorSpaceEditField())
               ->setKey('spacePHID')
               ->setLabel(pht('Space'))
@@ -114,7 +117,7 @@ final class PhabricatorPolicyEditEngineExtension
               ->setConduitDescription(
                 pht('Shift the object between spaces.'))
               ->setConduitTypeDescription(pht('New space PHID.'))
-              ->setValue($object->getSpacePHID());
+              ->setValue($space_phid);
             $fields[] = $space_field;
 
             $space_field->setPolicyField($policy_field);

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

@@ -527,6 +527,11 @@ abstract class PhabricatorApplicationTransaction
         // TODO: Remove this eventually, this is handling old changes during
         // object creation prior to the introduction of "create" and "default"
         // transaction display flags.
+
+        // NOTE: We can also hit this case with Space transactions that later
+        // update a default space (`null`) to an explicit space, so handling
+        // the Space case may require some finesse.
+
         if ($this->getOldValue() === null) {
           return true;
         } else {