Choose diff base in Changes sidebar

### Feature summary

Allow users to select what branch or commit is used as the base for the diff window

### What problem are you trying to solve?

Sometimes I want to see the diff to main on the upstream repo, sometimes I'm iterating on PRs and want to compare my local changes to what's on the remote. Often I make a branch off of origin/main but want to view the diff that would be shown if I made a PR off that branch. In 0.2.6 all I see is the diff from origin/main (maybe if I make a worktree off a different branch it would be based off that).

### Proposed solution

Maybe a dropdown in the 'Changes' sidebar that allows me to select what I want to compare to.

### Workflow impact

This would make reviewing agent's work much easier

### Installation context

_No response_

### Additional context

_No response_