git: Set files ref-backend for git-clone(1) and git-init(1)
When we initialize a repository via git-init(1), or clone a repository via git-clone(1), we don't specify the reference backend to use. This is OK, since Git, defaults to the files backend.
We eventually want to switch to the reftable backend. So let's add code
to make the reference backend explicit by specifying the --ref-format
flag.
In the upcoming commit, we'll add a feature flag to allow us to swap out the backend with the reftable backend.
Closes #6309 (closed)