In the 00s there was a popular wine called "old git". It generally means someone grumpy and cantankerous, but in a playful way (otherwise you'd just call them a c***). So it makes sense for a self-effacing joke about Linus Torvalds.
This is true. But I think there’s a surprising difference between calling someone ‘an old git’ / ‘a jammy git’ / ‘a grumpy git’ / … and simply calling them ‘a git’. The latter seems quite a lot harsher and less likely to be a joke.
They're exaggerating. It's a very casual insult, often used playfully
I thought so when I was younger, but I’ve found that it’s often considered worse than words like ‘idiot’ or ‘twit’. Some seem to find it — to my surprise — not just insulting, but crude.