Add support for bare git reference repos

This adds two parameters to the git module:

  bare (boolean)
    Indicates this is to be a bare repositori

  reference (string)
    Indicates the path or url to the reference repo.
    Check out the "--reference" option in the "git clone"
    man page

Added appropriate tests.
Merge pull request #4691 from cg-soft/git-bare-ref-repo-support

Add support for bare git reference repos