From: Konstantin Ryabitsev
Git doesn't use "master-slave" terminology -- the "master" comes from the concept of having a "master" from which copies (branches) are made:
https://simple.wikipedia.org/wiki/Master_recording
The concept predates the music business and goes back to middle ages when a guild master would create a "master work" or "master piece" that the apprentices could use for study or for imitation.