dirstate-item: point out that `merged` is set only with p1_tracked This is currently True, and we will use this fact to simplify the API in the next commit. However, we add this assertion first to validate that this is True in the whole test-suite.

dirstate-item: update the attribute documentation It was very outdated. We are about to change these attribute so we should has well have them documented so that the change get easier to grasp.