LOCAL/POLICY: Differential: Clear 'Depends On' when attaching new diff

When we attach a new diff to a Differential revision, clear out its
'Depends On' field. This is so we don't end up with dependency cycles
when we rebase/cherry-pick commits out of order.

As upstream do not use rebasing/chery-picking/multi-patch-branch
workflows, this is unlikely to be accepted.

src/applications/differential/editor/DifferentialTransactionEditor.php

@@ -160,9 +160,11 @@ final class DifferentialTransactionEditor
     $type_edge = PhabricatorTransactions::TYPE_EDGE;
 
     $edge_ref_task = DifferentialRevisionHasTaskEdgeType::EDGECONST;
+    $edge_deps = DifferentialRevisionDependsOnRevisionEdgeType::EDGECONST;
 
     $want_downgrade = array();
     $must_downgrade = array();
+    $kill_deps = false;
     if ($this->getIsCloseByCommit()) {
       // Never downgrade reviewers when we're closing a revision after a
       // commit.
@@ -171,6 +173,11 @@ final class DifferentialTransactionEditor
         case DifferentialRevisionUpdateTransaction::TRANSACTIONTYPE:
           $want_downgrade[] = DifferentialReviewerStatus::STATUS_ACCEPTED;
           $want_downgrade[] = DifferentialReviewerStatus::STATUS_REJECTED;
+	  // If the PHID is null, this is a newly-created revision, so there
+	  // are no old deps to kill.
+	  if ($object->getPHID()) {
+	    $kill_deps = true;
+	  }
           break;
         case DifferentialRevisionRequestReviewTransaction::TRANSACTIONTYPE:
           if (!$object->isChangePlanned()) {
@@ -198,6 +205,20 @@ final class DifferentialTransactionEditor
         ->setNewValue($want_downgrade);
     }
 
+    if ($kill_deps) {
+	$deps_to_remove = PhabricatorEdgeQuery::loadDestinationPHIDs(
+	    $object->getPHID(),
+	    $edge_deps);
+
+	if ($deps_to_remove) {
+	  $results[] = id(new DifferentialTransaction())
+	    ->setTransactionType($type_edge)
+	    ->setMetadataValue('edge:type', $edge_deps)
+	    ->setIgnoreOnNoEffect(true)
+	    ->setNewValue(array('-' => array_fuse($deps_to_remove)));
+	}
+    }
+
     $is_commandeer = false;
     switch ($xaction->getTransactionType()) {
       case DifferentialRevisionUpdateTransaction::TRANSACTIONTYPE: