Vendor branch

Imagine a scenario where you are already using Subversion to commit changes/updates to this internal project of yours. However, a problem occurs whenever a new version of WordPress? is released. You need to upgrade the WP core files without losing any of the custom modifications you’ve been making up until now. Ideally, you would like to apply (to your internal project) only the changes made to the upgraded WP core files without over-writing any of your own work. Solution:

“The solution to this problem is to use vendor branches. A vendor branch is a directory tree in your own version control system that contains information provided by a third-party entity, or vendor. Each version of the vendor’s data that you decide to absorb into your project is called a vendor drop.” (quote) So, in this case, the third-party entity or vendor, is WordPress? . What we want to do is: absorb the WP code into our own project.

http://techblog.touchbasic.com/html/using-subversion-with-wordpress-part-1-creating-vendor-branches-and-integrating-your-existing-code/