Git
The source files are stored using GIT (see https://lucene.apache.org/core/developer.html)
Subversion
The sourcefiles are now stored using Subversion (see http://subversion.tigris.org/ and http://svnbook.red-bean.com/)
Anonymous access
svn checkout http://svn.apache.org/repos/asf/lucene/dev/trunk lucene/dev/trunk
Commit access
See http://www.apache.org/dev/version-control.html for details on getting a username/password
svn checkout https://svn.apache.org/repos/asf/lucene/dev/trunk lucene/dev/trunk