tests: make test-subrepo-git.t compatible with git's master->main rename Git is about to change the default branch from "master" to "main". Recent versions has started warning about that, which makes tests fail. I assume the tests would fail in a different way once the default has changed. To make us compatible with that name change, let's set configure the default to be "master". That value makes the tests still work on older Git version (those where the default branch name is not configurable). Differential Revision: https://phab.mercurial-scm.org/D9746