Don't allow Herald Diff rules to "Call Webhooks"

Summary: Like "Commit Hook" rules, these also fire oddly and don't have an object PHID or a list of transactions.

Test Plan: Verified that "Call Webhooks" was no longer available from Diff rules, but still available from other rule types.

Differential Revision: https://secure.phabricator.com/D19069

src/applications/differential/herald/HeraldDifferentialDiffAdapter.php

@@ -57,4 +57,8 @@ final class HeraldDifferentialDiffAdapter extends HeraldDifferentialAdapter {
     return pht('New Diff');
   }
 
+  public function supportsWebhooks() {
+    return false;
+  }
+
 }