Extract everything after refs/heads/ as branch.

By dropping the front part of the ref, one is able to use branches with deeper nesting than 1. Since the subproject configuration allows this, it should be supported by the github helper method.

Example ref spec:
* master
* foo/bar
* foo/bar/baz

For documentation, see (Payload -> ref) at
https://developer.github.com/v3/activity/events/types/#pushevent