The PostgreSQL source code is stored and managed using the Git version control system. A public mirror of the master repository is available; it is updated within a minute of any change to the master repository.
Our wiki, http://wiki.postgresql.org/wiki/Working_with_Git, has some discussion on working with Git.
Note that building PostgreSQL from the source repository requires reasonably up-to-date versions of bison, flex, and Perl. These tools are not needed to build from a distribution tarball since the files they are used to build are included in the tarball. Other tool requirements are the same as shown in Chapter 16.