[JcrDiff] Root ItemInfo should use provided TreeItem childId as Id instead of always create one with index 0

JcrDiff (1.02.xx) ItemInfo now can and should honor the root NodeTree ChildId index instead of hard coding this to index 0.
A JCR backed ItemInfo its path then properly will reflect its SNS index in the repository, like: "/hippo:configuration[1]/frontend" instead of "/hippo:configuration/frontend"