bugfix: binary file panic

[asyncawaitpromise]

Summary / Background

A production PR with a binary file mode change caused codeowners-plus to panic when parsing the diff.

The git output for a mode change combined with a binary patch produces a FileDiff that the upstream go-diff parser leaves with empty OrigName and NewName. The old code then tried to slice the empty name and crashed.

This change recovers the filename from the diff --git extended header when the standard name fields are empty, and adds a regression test that pins the exact production diff shape.

Testing

Reproduced the panic against the original triggering diff. Parsed the raw diff through go-diff and confirmed the function on main panics with the exact stack trace from the ticket on the mode change plus binary patch entry.

Verified the fix. The same FileDiff that previously panicked now returns the correct filename instead of crashing.

Added a regression test using the exact production diff shape (synthetic file paths). Reverting diff.go to the previous version makes the test panic with the same stack trace, which confirms the test guards the right behavior. The full internal/git test suite passes.

[github-actions]

Codeowners approval required for this PR:

• ✅ @BakerNet or @zbedforrest

[gemini-code-assist]

Code Review

This pull request enhances the git diff parsing logic to correctly identify filenames for binary files by extracting them from the extended "diff --git" header when standard headers are absent. It also includes a fix for a potential panic during mode changes and adds several test cases to verify binary file handling, quoted paths, and renames. I have no feedback to provide.