Upgrade git on Debian/Jessie

Align git to v2.11.x (Debian/Stretch version) on the
older Jessie version as well.

Change-Id: Ia5cc94542c89dd96e5f953a815af3231f285ba03
diff --git a/jenkins-docker/slave-debian/Dockerfile b/jenkins-docker/slave-debian/Dockerfile
index 0dad8a8..a5ea735 100644
--- a/jenkins-docker/slave-debian/Dockerfile
+++ b/jenkins-docker/slave-debian/Dockerfile
@@ -28,10 +28,12 @@
     vim \
     && rm -rf /var/lib/apt/lists/*
 
+
 RUN [ "$DEBIAN_VER" != "jessie" ] || \
        ( echo "deb http://http.debian.net/debian jessie-backports main" > /etc/apt/sources.list.d/jessie-backports.list && \
          apt-get update && \
          apt-get install -y -t jessie-backports openjdk-8-jdk && \
+         apt-get install -y -t jessie-backports git && \
          update-java-alternatives -s java-1.8.0-openjdk-amd64 && \
          mv /etc/apt/sources.list.d/jessie-backports.list /etc/apt/sources.list.d/jessie-backports.list.disabled && \
          apt-get update && \