Fix two minor Differential issues

Summary: The JIRA field is currently always enabled. This isn't correct; it
should be disabled if there's no JIRA provider.

We also use the old set of reviewers to compute mail delivery. Instead, reload
the correct set of reviewers before moving on after finalizing transactions.

src/applications/differential/customfield/DifferentialJIRAIssuesField.php

@@ -13,6 +13,10 @@ final class DifferentialJIRAIssuesField
     return 'jira.issues';
   }
 
+  public function isFieldEnabled() {
+    return (bool)PhabricatorAuthProviderOAuth1JIRA::getJIRAProvider();
+  }
+
   public function canDisableField() {
     return false;
   }

src/applications/differential/editor/DifferentialTransactionEditor.php

@@ -504,6 +504,25 @@ final class DifferentialTransactionEditor
     PhabricatorLiskDAO $object,
     array $xactions) {
 
+    // Load the most up-to-date version of the revision and its reviewers,
+    // so we don't need to try to deduce the state of reviewers by examining
+    // all the changes made by the transactions. Then, update the reviewers
+    // on the object to make sure we're acting on the current reviewer set
+    // (and, for example, sending mail to the right people).
+
+    $new_revision = id(new DifferentialRevisionQuery())
+      ->setViewer($this->getActor())
+      ->needReviewerStatus(true)
+      ->withIDs(array($object->getID()))
+      ->executeOne();
+    if (!$new_revision) {
+      throw new Exception(
+        pht('Failed to load revision from transaction finalization.'));
+    }
+
+    $object->attachReviewerStatus($new_revision->getReviewerStatus());
+
+
     foreach ($xactions as $xaction) {
       switch ($xaction->getTransactionType()) {
         case DifferentialTransaction::TYPE_UPDATE:
@@ -527,19 +546,6 @@ final class DifferentialTransactionEditor
       case $status_accepted:
       case $status_revision:
       case $status_review:
-        // Load the most up-to-date version of the revision and its reviewers,
-        // so we don't need to try to deduce the state of reviewers by examining
-        // all the changes made by the transactions.
-        $new_revision = id(new DifferentialRevisionQuery())
-          ->setViewer($this->getActor())
-          ->needReviewerStatus(true)
-          ->withIDs(array($object->getID()))
-          ->executeOne();
-        if (!$new_revision) {
-          throw new Exception(
-            pht('Failed to load revision from transaction finalization.'));
-        }
-
         // Try to move a revision to "accepted". We look for:
         //
         //   - at least one accepting reviewer who is a user; and
@@ -551,7 +557,7 @@ final class DifferentialTransactionEditor
         $has_rejecting_reviewer = false;
         $has_rejecting_older_reviewer = false;
         $has_blocking_reviewer = false;
-        foreach ($new_revision->getReviewerStatus() as $reviewer) {
+        foreach ($object->getReviewerStatus() as $reviewer) {
           $reviewer_status = $reviewer->getStatus();
           switch ($reviewer_status) {
             case DifferentialReviewerStatus::STATUS_REJECTED: