`/diff` allows you to choose base branch for `branch diff` view

Describe the feature or problem you'd like to solve

In Copilot CLI, the /diff command should allow you to be able to set the base branch when viewing in branch diff mode vs local edits.

Proposed solution

If I'm working on branch b with the following format:
develop -> branch a (feature) -> branch b (jira task)
I want the branch change mode for /diff on branch b to reflect the branch diff vs branch a, not automatically determined develop.

Example prompts or workflows

/diff set-base <branch>

Additional context

No response

Start at the Copilot CLI implementation of the /diff command and its branch diff mode. Trace how the base branch is currently determined, then assess the proposed /diff set-base <branch> workflow. Done means branch diff mode can use the explicitly selected base branch instead of the automatically determined branch, while local edits remain distinct.