buildserver: use git from jessie-backports to get better workflow

A number of key features around username/password handling where added in
2.3, like GIT_SSH_COMMAND and GIT_TERMINAL_PROMPT.

buildserver/provision-apt-get-install

@@ -50,8 +50,8 @@ packages="
  flex
  gettext/jessie-backports
  gettext-base/jessie-backports
- git-core
- git-svn
+ git-core/jessie-backports
+ git-svn/jessie-backports
  gperf
  graphviz
  imagemagick