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
f534ca2e8a