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
Merge pull request #497 from nblock/bugfix/ref-extraction

Extract everything after refs/heads/ as branch.