On installing the macOS Catalina update, I realised that in Visual Studio Code Git is completely missing and cannot be added.

The problem is apparently XCODE.  Instructions for updating are here:

https://stackoverflow.com/questions/52522565/git-is-not-working-after-macos-update-xcrun-error-invalid-active-developer-pa

But, should that link go, then go into Terminal, run this code and follow the update instructions:

xcode-select --install

Once it is done, restart your Mac and Git should be back in place in the Source Code section of Visual Studio Code.