branch city improvements

[koschke]

We want to answer the following questions a user of BranchCity may have:

• What are the exact changes from the previous revision to the latest revision? We want to see an animation of the transition of the two graphs in the same way as EvolutionCity renders it.
• Which files are modified by different authors? The files with conflicting changes should be more prominently highlighted.
• Which file is on which branch?
• What exactly has been changed by an author in a particular file or commit?