repo-toolset doesn't support git working trees

I'm trying to use `git worktree` to segregate some work in dotnet/project-system without having to manage constant stashing or temporary commits, but running into an issue where `GitHash.props` assumes that the `.git` path is `$(RepoRoot).git`.

Locally to work around this have been able to define two properties to allow optionally overriding the path to the `.git` directory, and the directory where the HEAD file is located, but its not really an ideal solution. It should be possible to define one property to override the path to `.git` and another for `IsWorkTree`, then the location of the HEAD file could be derived from there, but ideally if it was possible to get the commit sha by calling git directly, none of that would be necessary.