As git_clone now has callbacks to configure the details of the
repository and remote, remove the lower-level functions from the public
API, as they lack some of the logic from git_clone proper.
If requested, git_clone_local_into() will try to link the object files
instead of copying them.
This only works on non-Windows (since it doesn't have this) when both
are on the same filesystem (which are unix semantics).