Getting rid of "master" and "slave" I can understand. If I was the only black kid in computer science class and they started to talk about "master" and "slave" I might feel uncomfortable and put off.

But it's not clear to me that git master has the same sort of meaning. Why couldn't it have a meaning more like 'master copy'?

master copy

an original copy, stencil, tape, etc, from which duplicates are made