When using the GitHub Organizational Folder plugin, we have the option to Build origin PRs (unmerged head). I would love to use this feature, however in the Pipeline job that gets created and run for our PR's, I'm unable to find any way to get the branch name, which is absolutely necessary for various parts of our build process. 

The ideal methods...

env.BRANCH_NAME // returns PR-1234
scm.branches[0] // returns PR-1234

The often suggested hack, post-checkout...

sh(returnStdout: true, script: 'git rev-parse --abbrev-ref HEAD') // returns HEAD

...returns "HEAD" and I'm not entirely sure why.

Am I completely missing something here? This just seems like it would be painfully necessary for some people. Using `env.BRANCH_NAME` does work for the ordinary branch/non PR jobs. I found several older JIRA issues related to that, but cannot find anything related to the PR builds specifically.