Webhooks for branching in Gitlab

how can one get a Webhook to fire when a particular ~branch~ changes in the repository?

That looks like issues 1176

If you look at the posted data, you have

:ref => "refs/heads/master",

Webhooks works sending a POST request to a script, with a payload describing the push.

You need a service listening to that webhook though.

If you use a Jenkins server, you can have a Jenkins GitLab plugin "branch filtering" for your webhook to point to.