Handle edge-cases in diff'ing

[cloudhead]

There are two edge cases we need to handle, or make sure we handle correctly, potentially via a simplified commit-diff function:

1. Merge commits. We need to have an easy to to diff the two trees on the target branch (eg. master)
2. Initial/root commits. We need to show something correct when showing the diff of the first commit of a repo.

These could be handled via a Browser::commit_changeset() function, for example.