Fix git diff to compare against HEAD instead of working directory

[adboio]

TL;DR

Changed git diff operations to compare against HEAD instead of the working directory, ensuring we correctly identify staged vs unstaged file changes.

What changed?

• Modified getChangedFilesDetailed function in packages/git/src/queries.ts to use git.diff(["HEAD", "--", file]) instead of git.diff([file])
• Added checks to skip files when the diff against HEAD is empty (indicating no changes relative to HEAD)
• Applied this change to both the unstaged changes section and deleted files section of the diff logic
• Wrapped the HEAD diff check in a try-catch block for the deleted files case to handle potential errors gracefully

How did you test this?

Unable to verify testing details from the provided information.

[adboio]

• feat(code): persist PR URLs to backend & emit file activity events #1434
• feat(code): add attribution to PR body from UI #1433
• feat(code): add commit and pr attribution to system prompt #1432
• feat(code): add commit trailers for attribution #1431
• Add staged file tracking and selective commit support #1429
• Fix git diff to compare against HEAD instead of working directory #1422 👈 (View in Graphite)
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