How do I get Quickly to use Git as its default version control instead of Bazaar?
Unfortunately, in the current state of Quickly, you cannot. However it maybe possible to have Quickly extended to support this in the future, and you could report a bug.
I would caution against it though, since Quickly is not designed to use git, as it's really a tool for making it easier to use the Ubuntu related tools and environment to build Applications for Ubuntu (and possibly other Linux/UNIX systems if you choose to port the applications later). These tools would include Launchpad, Bazaar, etc.