gistfile1.el

(defun github-browse ()
  (interactive)
  (let ((url (magit-get "remote" (magit-get-current-remote) "url")))
    (save-match-data
      (if (string-match "github.com:\\([^/]+\\)/\\([^\.]+\\).git" url)
          (let ((owner (match-string 1 url))
                (repo (match-string 2 url)))
            (browse-url (format "https://github.com/%s/%s/blob/%s/%s#L%d"
                                owner repo
                                (magit-get-current-branch)
                                (magit-file-relative-name (buffer-file-name))
                                (line-number-at-pos))))))))