Better structure for DiffTree so we traverse up the chain

[Cody Mikol]

Should be

DiffTree: {
    fileDeltaNodes: List<FileDeltaNode>
}

FileDeltaNode: {
  parent: DiffTree
  fileDelta: FileDelta
  hunkNodes: List<HunkNode>
}

HunkNode {
  parent: FileDeltaNode
  hunk: Hunk
  lineNodes: List<LineNode>
}

LineNode {
  parent: HunkNode
  line: Line
}