(require 'dired-x)

(defvar dired-omit-regexp-orig (symbol-function 'dired-omit-regexp))

(defun dired-omit-regexp ()
  (let ((file (expand-file-name ".git"))
        parent-dir)
    (while (and (not (file-exists-p file))
                (progn
                  (setq parent-dir
                        (file-name-directory
                         (directory-file-name
                          (file-name-directory file))))
                  ;; Give up if we are already at the root dir.
                  (not (string= (file-name-directory file)
                                parent-dir))))
      ;; Move up to the parent dir and try again.
      (setq file (expand-file-name ".git" parent-dir)))
    ;; If we found a change log in a parent, use that.
    (if (file-exists-p file)
        (let ((regexp (funcall dired-omit-regexp-orig)))
          (assert (stringp regexp))
          (concat
           regexp
           (if (> (length regexp) 0)
               "\\|" "")
           "\\("
               (mapconcat
                #'(lambda (str)
                    (concat "^"
                             (regexp-quote
                              (substring str 13
                                         (if (= ?/ (aref str (1- (length str))))
                                             (1- (length str))
                                           nil)))
                             "$"))
                  (split-string (shell-command-to-string
                                 "git clean -d -x -n")
                                "\n" t)
                  "\\|")
               "\\)"))
      (funcall dired-omit-regexp-orig))))