Don't run Herald when applying inverse edge transactions

Summary: Fixes T6727. Repro is: mention a task on another task, in a comment.

The inverse edge editor applying the "alincoln mentioned this in <other task>" transaction doesn't have enough data to execute Herald rules.

Just don't try to execute the rules, since they don't make much sesne from a product perspective and are tricky from a technical perspective.

Test Plan: Commented on `T1` with `T2` in comment body and a Herald rule that examines subscribers.

Reviewers: btrahan

NOTE: Cowboy committing this since any task mention fatals.

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -733,6 +733,13 @@ abstract class PhabricatorApplicationTransactionEditor
       // We are the Herald editor, so stop work here and return the updated
       // transactions.
       return $xactions;
+    } else if ($this->getIsInverseEdgeEditor()) {
+      // If we're applying inverse edge transactions, don't trigger Herald.
+      // From a product perspective, the current set of inverse edges (most
+      // often, mentions) aren't things users would expect to trigger Herald.
+      // From a technical perspective, objects loaded by the inverse editor may
+      // not have enough data to execute rules. At least for now, just stop
+      // Herald from executing when applying inverse edges.
     } else if ($this->shouldApplyHeraldRules($object, $xactions)) {
       // We are not the Herald editor, so try to apply Herald rules.
       $herald_xactions = $this->applyHeraldRules($object, $xactions);