As found in subgit project mapping the configuration should be
[git "my-subproject"]
repository = /var/git/my-subproject.git
translationRoot = /
trunk = /my-subproject:refs/heads/master
branches = branches/project/*:refs/heads/*
shelves = shelves/project/*:refs/shelves/*
tags = tags/project/*:refs/tags/*
I hope this would be useful.