Cleanup whitespaces in a magit diff hunk

magit-cleanup.el

(defun davazp/magit-cleanup-hunk-whitespace ()
  "Cleanup the whitespaces in the diff hunk under the cursor."
  (interactive)
  (let ((current (magit-current-section)))
    (when (eq 'hunk (magit-section-type current))
      (let ((file (magit-file-at-point))
            (context (caddr (magit-section-value current))))
        (cl-destructuring-bind (first-line count)
            (mapcar #'string-to-number (split-string context ","))
          (save-excursion
            (with-current-buffer (find-file-noselect file)
              (let (start end)
                (goto-char (point-min))
                (forward-line (1- first-line))
                (setq start (point))
                (forward-line (1- count))
                (setq end (point))
                (whitespace-cleanup-region start end)
                (magit-refresh)))))))))

Bind it with

  (define-key magit-hunk-section-map (kbd "w")  'davazp/magit-cleanup-hunk-whitespace)