magit-shallow.el

(require 'magit-process)

(defun shallow-checkout (branch &optional directory)
  (interactive (list (magit-read-string-ns "Branch") default-directory))
  (unless (file-exists-p (expand-file-name branch directory))
    (magit-run-git "clone" "--branch" branch
		   "--single-branch" "--shared"
		   (expand-file-name "git" directory)
		   (expand-file-name branch directory))))

(defun shallow-clone (repository directory)
  (interactive
   (let  ((url (magit-read-string-ns "Clone repository")))
     (list url (read-directory-name
                "Clone to: " nil nil nil
                (and (string-match "\\([^/:]+?\\)\\(/?\\.git\\)?$" url)
                     (match-string 1 url))))))
  (setq directory (file-name-as-directory (expand-file-name directory)))
  (mkdir directory)
  (magit-run-git-async "clone" "--bare" repository
		       (magit-convert-filename-for-git
			(expand-file-name "git" directory))))