find-git-dirs.rkt

(define (find-git-dirs path)
  (define (basename path)
    (let-values ([(base name must-be-dir?) (split-path path)])
      (match name
        ['same (string->path ".")]
        ['up   (string->path "..")]
        [name  name])))
  (define .git (string->path ".git"))
  (find-files
    (λ (path)
       (and (equal? (file-or-directory-type path) 'directory)
            (equal? (basename path) .git)))
    path))