While it was never difficult to use branch names other than "master" for the mainline development trunk in a repository, it was far and away the convention. As people have started to think about the language used in our tooling, more and more people are likely to move to "main" or some other name. Git has added a new config option to make this easier for initializing new repositories. RBTools would usually pick up non-default names for tracking branches already, since it uses git's "branch.X.remote" config keys. However, that doesn't work for bare repositories where there's no HEAD, and in the case that those config keys don't work for some reason, we still want a more correct fallback. This change adds that test to look up the configured default branch name and try that first, and then finally just returning "origin/master". While I was doing this, I also fixed up the git-dir detection when the current repo is a worktree created from a checkout elsewhere. This didn't actually impact anything since the git-dir usage that would have been impacted only applied to git-p4 checkouts which are highly unlikely to have worktrees, but it makes things more correct. Testing Done: - Ran unit tests. - Manually verified new git-dir detection behavior with my local worktree.