ENV: get source branch

for using https://github.com/tir38/android-lint-entropy-reducer, I need the source branch to push the result of “android lint” (code quality scanner) to the same branch.
I do only get the target branch (which is in my case ‘master’), but I would need the source branch of the pull request on github (e.g. ‘testLint’).
Would it be possible to add this? Or is there another way to get this information?
According to the script this is done in travis with ENV[‘TRAVIS_BRANCH’]

Thank you for this great software.