Message from 2021
This post is pretty old! Opinions and technical information in it are almost certainly oudated. Commands and configurations will probably not work. Consider the age of the content before putting any of it into practice.
Learned this today when setting up an fsfs repository; if users are going to be using svn+ssh or some other method that reads and writes to the database as their uid, you need to make the permissions setgid:
cd repodir chmod -R g+s .
What this does is makes any new files in the directory take the group of the directory, and not the primary group of the user that created them.