I'm old enough to remember the SVN days (he'll, even the CVS and....dare I say it.... source safe days).
Git is fantastic. It's pretty universally uses because it's the best dvcs out there and it's free. It wipes the pants with the likes of mercurial.
In certain industries (such as gaming) there's still a strong hold by perforce but we can ignore that as it's proprietary and a bit specialised.
Anyway, as great as git is for making things easier and cleaner when dealing with distributed development, it by no means makes something impossible "possible" - it just makes it a hell of a lot easier.
The Linux kernel on the other hand enabled a lot of impossible things. Remember back in the day there wasn't anything free and open source in the operating system world, it was all proprietary and licensed. If you wanted to create your own operating system, you basically had no option but to spend a fortune either writing your own kernel or licensing someone else's (and the licensing part means you cannot distribute it for free).
The fact that the FSF has always wanted to write their own OS and never been able to achieve it without the Linux Kernel, in spite of them essentially writing "everything else" that makes up an operating system, shows just how nontrivial this is.