Clone the repository:
git clone --recursive https://github.com/geonetwork/core-geonetwork.git
And build:
cd core-geonetwork
mvn clean install -DskipTests
GeoNetwork use submodules, these were initiziled by the --recursive option when cloning the repository.
If you missed using --recursive run the following:
cd core-geonetwork
git submodule init
git submodule update
Submodules are used to keep track of externals dependencies. It is necessary to init and update them after a branch change:
git submodule update --init
Remember to rebuild the application after updating external dependencies.