Yeah so there is a master/slave model but I agree that that's not what GitHub was using. I tried to clarify this with my parenthetical statement but apparently did not do so sufficiently. Any suggestions of how to edit?

> A master branch evokes mastery of a subject, like a Masters degree.

I would actually say that that a Master degree is using a different definition (though both part of the adjective usage). For a master branch (or master document, master copy) I'd say it is the definition that is "main" or "principal". Whereas a Masters degree is having mastery over something, which is akin to high proficiency.