Derive correct name of BAM file based on batching.

def _merge_outfile_fname(out_file, bam_files, work_dir, batch):
    """Derive correct name of BAM file based on batching.
    """
    if out_file is None:
        out_file = os.path.join(work_dir, os.path.basename(sorted(bam_files)[0]))
    if batch is not None:
        base, ext = os.path.splitext(out_file)
        out_file = "%s-b%s%s" % (base, batch, ext)
    return out_file