The problem with the linked article in your question is that it is suggesting to download WordPress and then create your own git repo – but then this cannot be easily updated. Instead you can have WordPress itself in git. Rather than the default subversion, there is an offical Github repository of WordPress, and in the same fashion as with subversion tags you just want to clone the latest branch.
Personally I manage things through subversion (but the principles can be converted to Git):
- I have a subversion checkout of the current WordPress tag on the server and on my local machine.
- Follow the WordPress guide on tracking stable versions
- Plugins are not under source control – I don’t really see the benefit if you’re not modifying the plugins yourself and its easier to just let WordPress handle those
- My theme is under subversion source control
- It is perfectly possible in subversion to have one repository inside another then you can update the core independently from the theme