graft: explicit current node tracking This changes graft to explicitly track the progression of commits it makes, and updates it's idea of the current node based on it's last commit, rather than from the working copy parent. This should have no effect on the value of current since we were reading the working copy parent immediately after commiting to it. The motivation for this change is that a subsequent patch will break the current node and working copy relationship. Splitting this out into a separate patch will make that one more readible.