Optimize cloning Git repositories for the refname checkout strategy

[Sanskar Jaiswal]

At the moment, cloneRefname() does a git ls-remote and uses the output to figure out the commit hash related to the provided refname. It then simply calls cloneCommit() with the obtained commit hash. While this works and is straightforward, it has the drawback of performing a full clone and not a shallow clone, which can be be a considerable performance hurdle. To avoid this, we should figure out if the provided refname points to a branch/tag and then perform a shallow clone like we do in cloneBranch() and cloneTag()