[Feature Request] Allow to review specific commits or commit ranges

# Motivation

The tool works great for reviewing PRs.
I can submit my comments to the agent once it's done with a piece of work.

Though, there is no good way for reviewing the iterations the agent does on PR.

I imagine the flow can be something like this:
1. An agent creates a PR
2. I review it with plannotator, leave some comments
3. The agent addresses them.
4. I review the changes only. And either accept or leave some more comments and go to step 3.

When I ask the agent explicitly to show me only the changes that it did - it opens a PR changes for me again.
Or tries to come up with some work around, like creating a separate branch with only the changes that I want to review.