Git is not equivalent to retard. Git is an everyday word. You can definitely get away with calling a pal a grumpy git. You're not going to get away with calling peoe retards unless you know them very well. And don't do so in public.
And 100 years ago you could get away with calling someone who was mentally impaired a "retard". That's the wonderful thing about language, it's always evolving.
Personally, if someone called me a "git" or a "retard" I would take it exactly the same way, since they are both connotations for "idiot" or "slow thinker".
yes, language evolves. Now we recognise that retard was used to define people as sub-human and then to murder them; experiment upon them; forcibly sterilise them against their will and without their knowledge; deny them medical treatment (which often leads to their slow painful death); to bully and harass them; deny them employment; deny them opportunity; segregate them; abuse them; rape them.
Using the word retard against people who do not have LD is a weak insult to them, but does cause hurt to a large number of weak, dis-empowered people.
Retard as an insult is fucking stupid and lazy because it misses the mark.
At least, this is the case for the UK.