you can also use GIT_BRANCH variable as appears here: https://wiki.jenkins-ci.org/display/JENKINS/Git+Plugin
The git plugin sets several environment variables you can use in your scripts:
GIT_COMMIT - SHA of the current
GIT_BRANCH - Name of the branch currently being used, e.g. "master" or "origin/foo"
GIT_PREVIOUS_COMMIT - SHA of the previous built commit from the same branch (the current SHA on first build in branch)
GIT_URL - Repository remote URL
GIT_URL_N - Repository remote URLs when there are more than 1 remotes, e.g. GIT_URL_1, GIT_URL_2
GIT_AUTHOR_EMAIL - Committer/Author Email
GIT_COMMITTER_EMAIL - Committer/Author Email