Patch Set 1: I would prefer that you didn't submit this

I am not fully happy with the state of the merge, but basic
tests work and I want to share early how the merge is intendet
to look like.
The main work is done in ResolveMerger and MergeCommand. I would be thankful for comments on these classes.

Missing are still a lot of tests, especially for error situations. Rename detection is also missing yet.