At work, we have a codebase that is used by numerous projects. The solution for project specific changes is 'checkout the repo, and make your changes locally'.

At work, we have a codebase that is used by numerous projects. The solution for project specific changes is 'checkout the repo, and make your changes locally'. What that has turned into is 'checkout the repo and make a new directory structure that symlinks everything back to the original'. The keeps the maintenance burden relatively small as you can easily update the base version, and only need to worry about the files you changed. I certainly wouldn't recommend using this approach for anything; but it is not as terrible as it sounds.