mtn2git: Add an example script that can be used to update the git mirror

mtn2git: Add an example script that can be used to update the git mirror
    This is an example as it is using relative paths and needs some changes
    to work for you.
    It assumes that you have a git repository (git-init was ran) and then
    will update it there. Before each update it will clone the git repository
    and copy the old state files into the repository. So in case something
    goes wrong you can do a post mortom. The state files are relatively small
    and the git repository is working with hardlinks so it does not even take
    too much space.
    On first pull you need to run git-repack or git-gc