I wrote an app to let git host mono repos and have permission settings. The app is composed by client and server sides, which are decoupled. The client is setting sparse-checkout rules, but will filter dir and files by server's auth response. The server is a proxy layer through upstreams like github/gitlab, but with per user's permission settings, which is absent in git world

Link: https://github.com/zhuzhonghua/blackgit