Refactor SCM.get_latest
    
    git-ls-remote accepts paramter refs to just return refs for specific
    ones. This refactor uses this parameter to avoid handling full list of
    refs from remote repository.
    
    In original code, option --exit-code is passed to git-ls-remote, which
    would cause command return exit code 2 when no ref is found from remote
    repository. SCM._run raises an error if that happens, but the raised
    error is not handled. This refactor catches this error to ensure the
    original behavior happens, that is if a ref is not found, treat it as a
    commit hash and call SCM.get_full_commit_hash.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>