[FEATURE] configurable base ref for the working-tree diff indicator.

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The diff stat shown in the UI appears to compare against a fixed base (the repo's main branch), which folds pre-existing commits and untracked files into the count. On a branch cut from develop (identical to develop but ahead of master), a clean session showed +432/−10 — entirely pre-existing commits plus one untracked doc, none of it changed this session.

Proposed Solution

Please allow configuring what the indicator diffs against, e.g. the branch's upstream/base (@{upstream} or the merge-base with a configurable ref) rather than always master/main, and optionally a toggle for whether untracked files are included.

Alternative Solutions

_No response_

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

_No response_