Add tracking submodules option

Requires git>=1.8.2 to function correctly.  Silently fails if not the required version.

After https://github.com/jenkinsci/git-client-plugin/pull/122 gets integrated, it will have submodule tracking support.  This pull request surfaces this option to Jenkins projects.  It enables your repo to always update (git submodule update --remote) to the tip of the branch that you configured it with (git submodule add -b 'branch' 'url').

This is useful where you have a "configuration" project that has a bunch of files that are common to a large number of projects, but don't want to duplicate.  You add this "configuration" repo to your repo as a submodule (git submodule add -b 'branch' 'url'), and then update the submodule with --remote to get the latest SHA-1 on the configured branch & update the local copy.