Build intermediate data structure before building merged Spoon tree

[slarse]

This will accomplish two things:

1. It allows Spork to resolve the left and right sides of conflicts at an earlier stage.
2. It allows Spork to restart earlier in case a conflict that can't be handled (say, a move conflict) is detected.
3. It simplifies actually building the tree.
4. It allows Spork to just clone unaltered parts of the revisions instead of incrementally building them one node at a time, which should ultimately lead to significant performance gains.

I won't sacrifice too much time here, but pulling this off would make for a much cleaner architecture that's easier to describe in the thesis.