Allow checkout_branch to point to a tag in clone_repository method

When I try to provide a tag to the checkout_branch flag in pygit2.clone_repository(), it fails and is unable to find the reference. It would be nice to be able to checkout a tag right away (like a branch) on clone to clone a specifc release of a package

Start at the pygit2.clone_repository entry point and trace how checkout_branch is resolved. Compare tag handling with the existing branch behavior; done means cloning can select a specified tag immediately, with the behavior covered alongside the existing clone tests.