Git is a fairly mild insult though, roughly equivalent to calling someone annoying. I'm sure at least a few of us have thought Git (the tool) to be aptly named, from time to time.

It was also equivalent to "bro" in the late 90s, at least in some circles in the US.

It's pretty mild, but still offensive in the UK. Your friend was right to believe that much. That everyone thinks it means that? No - the US uses it differently for a region of the body slightly less offensive. It's a bit like spunk, which is also fairly offensive in the UK, and fag, which despite having an offensive meaning in the US, is actually traditionally not offensive here, though the US meaning is known and sometimes used - it means "cigarette" here mostly, and "faggot" means something less offensive too (a bunch of sticks or a type of meatball like dish.)

Might be regional but fanny is definitely slang in the US as well, but very quaint/dated, meaning butt. Would generally be used in some sort of context like a grandma telling a kid, 'Get your fanny over here right this second!'

It would never be offensive or used with sexual connotation. It's kind of like the equivalent of wiener.

It was a popular name in France in the 90s ¯\_(ツ)_/¯