From phpBB Development Wiki
The version control system for phpBB4 is git.
There is a public git repository that serves as the source for releases and as a point to develop against. This repository is managed by the release manager. Every developer has their own public repository to which they push their changes. The release manager decides which of these changes & additions to merge into the main repository.
Patches from regular users can be picked up by any other user or developer. They should be audited closely. Since they also need to go through multiple layers until they reach the release manager quality concerns should be satisfied.
If the release manager should ever leave phpBB someone else can simply take over managing that repository.
Setting up a system like github.com on code.phpbb.com might make sense.
We will use Bamboo for continuous integration.