git is not mainstream. it's mainstream on HN and for people who read blogs and care about their craft. This makes it a good indicator of a person's level. Not that you have to use git per se, but having used one of git/hg/bzr or at least hearing of them and trying them out would also do the trick.

What would you consider uber-hardcore?

Being able to give a list of weaknesses in something like git, a sensible discussions of the pro/cons of the approach taken by git, a comparison with the behaviors of other systems (with their pros and cons) and some idea for potentially improving it.

With the ubiquity of DVCSes and Github etc, I don't think there is an uber-hardcore level in source control. "Has used git etc in a team setting" is the closest I can find.

Implementing git.

That's well above the "O(log n)" level from the rest of the list. After all, "algorithms O(log n)" is not "you are Knuth".

Being proficient with git. That's on an entirely different level than just knowing how to use it.