Piston is a utility that eases vendor branch management. This is similar to svn:externals, except you have a local copy of the files, which you can modify at will. As long as the changes are mergeable, you should have no problems.
Piston has a similar purpose than svnmerge.py which you can find in the contrib/client-side folder of the main Subversion. The main difference is that Piston is designed to work with remote repositories. You should also investigate if SVK would be a better fit for you.
Many have learned the basics of using Git from the PeepCode Git screencast. In this PDF, Scott Chacon goes even further to explain the distributed filesystem behind the popular source code management system.