Improve Rebase handling

[haacked]

I'd like to be able to better visualize rebases. The trick is detecting that a rebase occurs.

One option is for SeeGit to keep its own hashes of each commit that doesn't include parents. That way when it detects new commits with different SHAs, but matches what SeeGit thinks is an existing commit, we identify it as a rebase and take the original commit, remove its parent, attach it to the tip, and update the SHAs of those commits to the new ones.

Not sure how well this will work. Open to new ideas.