Add support for bare git repos

This adds a "bare" boolean parameter to the git module to create bare repos. I tested this in our own setup, but I'm not completely happy with it, mostly because I'm not sure I like the way the before/after stuff is done, and I'm also worried about the fetch refspecs for bare repos.

Consider this a discussion item. I do need bare git repo support for our environment, and we have a hacky solution in place now, but it would be great if we can have it out of the box.