[BUG] Desktop app shows massive incorrect diff for worktree branch at same commit as base

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The Claude Code desktop app shows a massive diff (+595,528 -28,470) in the PR creation bar for a worktree branch (claude/pensive-bhabha) that is at the exact same commit as master. The diff should be zero.

The UI shows master ← claude/pensive-bhabha with the large diff numbers and a "Create PR" button, even though both branches point to the same SHA (592dbf269).

Verified via CLI:

$ git rev-parse HEAD
592dbf269351c2314c227980ab554ddcc7bc868e
$ git rev-parse origin/master
592dbf269351c2314c227980ab554ddcc7bc868e
$ git diff origin/master --stat
(no output — zero diff)

The worktree branch has no remote tracking branch set, which may be relevant.

What Should Happen?

The diff should show +0 -0 (or hide the PR bar entirely) when the worktree branch is at the same commit as the base branch.

Error Messages/Logs

No errors — just incorrect diff numbers in the UI.

Steps to Reproduce

1. Open Claude Code desktop app
2. Start a conversation with --worktree (or let it auto-create a worktree)
3. Do not make any changes — the worktree branch is created at the same commit as master
4. Observe the PR creation bar at the top showing a massive diff

Claude Model

Opus

Is this a regression?

Not sure

Last Working Version

_No response_

Claude Code Version

Latest

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

The repo's default branch is master (not main). The worktree branch claude/pensive-bhabha has no upstream tracking branch set. The diff numbers (+595,528 -28,470) look like the entire repo contents being counted as new, suggesting the diff computation may be failing to find a common ancestor or is comparing against an empty tree.