int checkout_count_callback(
    git_checkout_notify_t why,
    const char *path,
    const git_diff_file *baseline,
    const git_diff_file *target,
    const git_diff_file *workdir,
    void *payload)
{
    checkout_counts *ct = payload;

    GIT_UNUSED(baseline); GIT_UNUSED(target); GIT_UNUSED(workdir);

    if (why & GIT_CHECKOUT_NOTIFY_CONFLICT) {
        ct->n_conflicts++;

=>      if (ct->debug) {
=>          if (workdir) { //false
               ......
            } else {
=>              if (baseline) {//false
                    ......
                } else {
=>                  if (target)//false
                        ......
                    else
                        fprintf(stderr, "How can a nonexistent file be a conflict (%s)\n", path);
                }
            }
        }
    }

    if (why & GIT_CHECKOUT_NOTIFY_DIRTY) {
        ct->n_dirty++;

        if (ct->debug) {
            if (workdir)
                fprintf(stderr, "M %s\n", workdir->path);
=>          else 
=>              fprintf(stderr, "D %s\n", baseline->path);
        }
    }

    .......
}