Fix undefined branch in create-destination-dialog

Follow-up of change 234145, where I think this dialog was just
overlooked.

As in all the other dialogs the bare confirm event from gr-dialog must
be prevented from propagating, so that the new detailed event is the
only one handled by parent elements.

Otherwise two 'confirm' events would be handled, and the one from
gr-dialog does not have the 'branch' detail, so it would become
undefined in the dialog.

Bug: Issue 12024
Change-Id: I8374e9dbd27ec69972011e3cfed58acb0f7052d4
(cherry picked from commit f534ca2e8a1a33251ee17229ec18a3b424e4429a)
diff --git a/polygerrit-ui/app/elements/change-list/gr-create-destination-dialog/gr-create-destination-dialog.js b/polygerrit-ui/app/elements/change-list/gr-create-destination-dialog/gr-create-destination-dialog.js
index d601cad..c2bfbf5 100644
--- a/polygerrit-ui/app/elements/change-list/gr-create-destination-dialog/gr-create-destination-dialog.js
+++ b/polygerrit-ui/app/elements/change-list/gr-create-destination-dialog/gr-create-destination-dialog.js
@@ -46,9 +46,13 @@
       this.$.createOverlay.close();
     },
 
-    _pickerConfirm() {
+    _pickerConfirm(e) {
       this.$.createOverlay.close();
       const detail = {repo: this._repo, branch: this._branch};
+      // e is a 'confirm' event from gr-dialog. We want to fire a more detailed
+      // 'confirm' event here, so let's stop propagation of the bare event.
+      e.preventDefault();
+      e.stopPropagation();
       this.dispatchEvent(new CustomEvent('confirm', {detail, bubbles: false}));
     },