This is mentioned elsewhere, but the connotation of master in the context of source control seems to be more along the lines of master used in terms of artifact creation. For example, where you have a master copy of a film or album. This is quite different from the definition of master in master/slave terms. This is definition 27 on dictionary.com.
of or relating to a master from which copies are made: master film; master matrix; master record; master tape.
https://www.dictionary.com/browse/master