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.

Ran unit tests.
Manually verified new git-dir detection behavior with my local

  worktree.