test-driven development versus Git

These are two completely separate and different things, I can't see how one makes the other redundant.

You use git to be able to go back to any point in your project, see changes, enable peer review, facilitate working with other developers and store a backup copy of your project somewhere. It also lets you do experiments with features on separate branches so that they don't interfere with the main working version.

TDD is a methodology where you first write tests and then the code that makes those tests pass. And you write them before implementing any new feature. In classic methodology you write tests after writing the feature or even after completing the whole project (depends if you're using Agile or Waterfall model). In the future those tests are used to document your application and you run them to check if any new changes broke something in the app (yes, this can as well be achieved by writing tests AFTER writing the code).