Implement a proper unparse method for DiffTrees #70
Description
The old DiffTree.toTextDiff has been removed in #67 because it wasn't able to serialize DiffTrees with moved subtrees. We need a replacement that is able to cope with all DiffTrees properly.
In contrast to line graphs the resulting format should be human readable (e.g. for use in debug messages) and ideally injective to be used in test assertions.