Do not hard-code branch name `master` for git-based repositories

[LinqLover]

'master' is still hard-coded somewhere in the system currently the default branch name will still be replaced by master.

Originally posted by @LinqLover in https://github.com/Metacello/metacello/issues/537#issuecomment-857800893

[macta]

agree that with the move to main replacing master, its annoying that you have to prefix everything with main: now