The `data-barba-prevent` attribute doesn't work on a `self` transition

[xavierfoucrier]

When using a prevent data attribute on a link that run a self transition, the transition is not prevented.

Note that the prevent strategy work as expected, so currently you can prevent the transition from running by using event.preventDefault() followed by event.stopPropagation() inside the prevent callback.

[xavierfoucrier]

Re-introduced by 6d35aa4.