I've just downloaded Jenkins 1.434 to see wether the last comment in #
JENKINS-7554 from Christian Weiske is true that the newest git plugin solves the problems with the GIT_BRANCH/GIT_COMMIT envvars and setting the build up to build multiple branches.
Unfortunately it turns out that the plugin does not support building multiple branches anymore at all. No matter wether I leave the branch field empty or add multiple branches to the build configuration it always only builds master. Tried with a totally fresh job so all branches need to be considered 'updated' and polling as well as manual building.