Jenkins Github Plugin - Triggering a Parameterized Build

问题

I have a "Freestyle project" Jenkins job that has a single parameter--GIT_BRANCH so that I can trigger it manually or via a GitHub hook.

The problem is that the GitHub plugin doesn't seem to correctly identify / set the branch that was pushed.

When I look at the "GitHub Hook Log" view on the Jenkins build, I see (timestamps don't match because I've resent the hook);

Last GitHub Push

Started on Jul 5, 2016 11:37:01 AM
Using strategy: Default
[poll] Last Built Revision: Revision 7c4xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (refs/remotes/origin/rc)
using GIT_SSH to set credentials 
 > git --version # timeout=10
 > git -c core.askpass=true ls-remote -h git@github.com:an-org/a-repo.git # timeout=10
Found 7 remote heads on git@github.com:an-org/a-repo.git
[poll] Latest remote head revision on refs/heads/rc is: 7c4xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx - already built by nnn
Done. Took 2.4 sec
No changes

Which is funny, because that's not the right branch at all, which I can confirm by looking at the GitHub hook log;

{
  "ref": "refs/heads/nil/jenkins-testing",
  "before": "0000000000000000000000000000000000000000",
  "after": "27eyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy",
  "created": true,
  "deleted": false,
  "forced": true,
  "base_ref": null,
  "compare": "https://github.com/an-org/a-repo/commit/27eyyyyyyyyyy",
  "commits": [
    {
      "id": "27eyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy",
      "tree_id": "acebbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
      "distinct": true,
      "message": "dummy commit",
      "timestamp": "2016-07-05T11:13:59-07:00",
      "url": "https://github.com/an-org/a-repo/commit/27eyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy",
      "author": {
        "name": "Peter Novotnak",
        "email": "peter@novotnak.net",
        "username": "pnovotnak"
      },
      "committer": {
        "name": "GitHub",
        "email": "noreply@github.com",
        "username": "web-flow"

The logs from com.cloudbees.jenkins.GitHubPushTrigger org.jenkinsci.plugins.github.webhook.WebhookManager and com.cloudbees.jenkins.GitHubWebHook (ALL) show absolutely nothing.

I've tried the following as the name of the git parameter in place of "GIT_BRANCH";