The official PostGIS code repository is a git repository hosted on
OSGeo ​Gitea instance (​https://git.osgeo.org/gitea/postgis/postgis).

To get the source from !OSGeo Git Service, and start a configure:

git clone https://git.osgeo.org/gitea/postgis/postgis.git
cd postgis
./autogen.sh
./configure

To fork PostGIS to modify and test new features, just click on the Fork button on top of the postgis page.
You'll need to login using your OSGeo Userid (a link to register a new account will be found on the login page).

A number of CodeMirrors do exist for users and contributor's convenience.