This is not direct answer to your question, but it is very bad practice to make changes to SharePoint system files located in file system (like, CORE15.css). It is much better to write separate CSS file, that could overwrite CORE15.css styles as you need. Then you can add it as alternative CSS to your master page in site collection settings.
Best option to store this separate CSS file is site collection Style Library or Master Page gallery.
About your question. When you work with Visual Studio TFS, all changes is made in your mapped workspace folder in your file system. And then you can make changes to files (that are usually in some Visual Studio projects), deploy them to SharePoint and later commit to TFS.