In recent years there has been a push by some to move away from using the term "master" in source code management systems, often by renaming the "master" branch to a "main" branch. This is part of a broader shift away from using "master/slave" terminology in software.
But it is often claimed that the "master" branch was never intended in a "master/slave" sense, but instead in a "master/replica" sense, like that of a master record. This certainly makes sense to me: I've never heard anyone talk about "slave" branches because they're not controlled by the "master" branch (unlike a "master/slave" server system), but an authoritative "master" branch you copy from in order to tinker with does make sense for how branches are used in SCMs. However I'm yet to see clear historical evidence that proves it.
So historically, in the context of source code management systems, what evidence is there that the earliest uses of "master" branches referred to either the master/replica or master/slave metaphor?
(Can we please stick to this historical question, and not rehash the present renaming debate here.)