Rename offensive terminology (master)

"master" is an offensive term, as it can be interpreted as being slavery-origin terminology. See https://en.wikipedia.org/wiki/Master/slave_(technology)#Terminology_concerns

The Python programming language, and various other projects, have taken a stance and moved away from offensive terminology including "master". See https://bugs.python.org/issue34605

When different projects using git decide to move away from "master" as the name of their main branch, inconsistency ensues between projects. See https://github.com/desktop/desktop/issues/6478 (and "Related Issues and Projects").

To avoid offensive terminology and to avoid further inconsistency, I think git should use a different branch name than "master" when initiating a repo. I don't have a strong opinion, but I like "main" since it shares the first two characters and it's shorter.