MemoryDiffCache should also check parent paths before falling to Loader (or returning null)

Each entry in MemoryDiffCache is keyed with (path, fromRev, toRev) for the list of modified children at path. A diff calcualted by DocumentNodeStore.diffImpl at '/' (passively via loader) or JournalEntry.applyTo (actively) fill each path for which there are modified children (including the hierarchy)

But, if an observer calls compareWithBaseState on a unmodified sub-tree, the observer will still go down to diffImpl although cached parent entry can be used to answer the query.