Don't show continuation lines in "stacked action" comment previews

Summary:
Ref T10698. Currently, we show the spacer/continuation lines around previews, but these don't make sense in previews.

(Other stuff also uses this code so I can't simply remove `spacer`.)

Test Plan:
Before:

{F1199924}

After:

{F1199925}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10698

Differential Revision: https://secure.phabricator.com/D15562

webroot/rsrc/js/application/transactions/behavior-comment-actions.js

@@ -148,10 +148,7 @@ JX.behavior('comment-actions', function(config) {
     } else {
       JX.DOM.setContent(
         JX.$(config.timelineID),
-        [
-          JX.$H(response.spacer),
-          JX.$H(response.xactions.join(response.spacer))
-        ]);
+        JX.$H(response.xactions.join('')));
       JX.DOM.show(panel);
     }
   }