DebianInstaller is a collection of many small Debian packages, that can each be checked out from its own git repository in the usual ways. This page explains how to check out the entire DebianInstaller source tree. DebianInstaller developers frequently checkout the whole tree for development.
While most of DebianInstaller is in git repositories, to check out the whole tree, you'll start with a checkout of the remains of our historical Subversion repository. Then the mr tool is used to check out the git repositories.
Anonymous checkout
svn co svn://svn.debian.org/svn/d-i/trunk debian-installer cd debian-installer scripts/git-setup mr -p checkout
Checkout over ssh, for developers
svn co svn+ssh://$userid@svn.debian.org/svn/d-i/trunk debian-installer cd debian-installer scripts/git-setup mr -p checkout
(replace $userid with your Alioth user ID)
You will need the version of mr in wheezy or later to use mr -p; in lenny or squeeze you can use mr -c .mrconfig checkout. With mr 1.00 and above, the "-p" is unnecessary. You may wish to add the -k option (accept untrusted SSL certificates when bootstrapping).
Simplified checkout, for developers
mr bootstrap http://anonscm.debian.org/viewvc/d-i/trunk/.mrconfig?view=co debian-installer
In order to make commits to the repositories, you'll have to request to be added to the d-i project group on Alioth. For this send a mail with an explanation to the project admins.

Note: svn://svn.debian.org/svn/d-i/trunk checked out as debian-installer/ top directory provides the complete mr bundle of all components of the DebianInstaller, while similarly named git://git.debian.org/d-i/debian-installer.git provides the installer/ component and is checked out later on by the mr tool.