Implement a proper unparse method for `DiffTree`s

[ibbem]

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.

[pmbittner]

@AlexanderSchultheiss You currently have an own implementation for unparsing in VEVOS right? Can you give us a pointer to where it is? Can we reuse it in DiffDetective?

[AlexanderSchultheiss]

I have not really implemented an unparse procedure yet. I only translate DiffTrees to the ground truth format of KernelHaven.