(defun vc-pijul--split-on-first-line (str)
  "Split STR on the first line.
That is, return a cons cell whose `car' is the first line of STR
and whose `cdr' is the rest of STR.
Note that newlines between the first line and the subsequent
lines will be removed.
If STR contains no newlines, just return a list with a single
element STR."
  (save-match-data
    (let ((first-newline (string-match "\n" str)))
      (cond
       (first-newline
        (cons (substring str 0 first-newline)
              (let ((rest (substring str (1+ first-newline))))
                (cond
                 ((string-match "\n+" rest)
                  (substring rest (match-end 0)))
                 (rest)))))
       ((list str))))))

  (let* ((args

  (let* ((support-description-p
          (with-temp-buffer
            (vc-pijul--call t "rec" "--help")
            (goto-char (point-min))
            (cond
             ((re-search-forward
               (rx-to-string
                '(seq bol (zero-or-more space) "--description")
                t)
               nil t)
              t))))
         (checkin-message
          (cond
           ((string-prefix-p "Summary: " comment)
            (substring comment 9))
           (comment)))
         (args

           (list "--all" "-m"
                 (replace-regexp-in-string "^Summary: " "" comment))

           (list "--all")
           (cond
            (support-description-p
             (let* ((splitted
                     (vc-pijul--split-on-first-line checkin-message))
                    (title (car splitted))
                    (body (cdr splitted)))
               (list "--message" title
                     "--description" body)))
            ((list "--message" checkin-message)))