As far as human beings, the problem was (and is) slavery, it's not the word "master". Especially in the US, there's this idea that we just have to change the terminology around, ban a word, and suddenly racism will disappear. Both political parties, the mainstream media, and major corporations all love this, because they get to make superficial changes and then sweep racism under the rug.
Now if GitHub wanted to do something truly radical to fight racism, they'd change the name of the "master" branch to the "black" branch. I understand most people will be resistant to this, simply because "black" has a negative connotation, but please be aware of your own personal reactions to the word, as that's my whole point. The word "black" also means darkness, evil, it's something dirty, while "white" means purity, goodness, and cleanliness. And I'm just pointing out the what's already in people's minds, don't shoot the messenger!
If something is "black and white", there's a clear right and wrong. In our language, "black" is synonymous with "wrong", and yet this word with negative connotations is used for an entire race of people.
Instead of practicing censorship, which is reprehensible in a free society, and reminiscent of the same kind of authoritarianism that produces master/slave relationships among human beings, why not do something actually progressive? Have we forgotten what that looks like? Challenge society's entire perception of the word "black", rename the "master" branch "black", and begin to associate the word with something good for once. Start by changing the way people think. Turn black into something positive, and if you really want to see people's racism come out, use white in a negative sense.