Add unit tests for getBranchName fallback behavior

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Task summary

Add focused unit tests for getBranchName() in src/git/diff.ts, including the fallback path that should return "unknown" when git commands fail.

Why this is a good first issue

The function is small, behavior is clear, and there is already an established test style in tests/git-diff.test.mjs to follow.

Suggested files or areas

• src/git/diff.ts
• tests/git-diff.test.mjs

Acceptance criteria

• Add at least one success test that verifies getBranchName() returns the current branch in a temp repo.
• Add at least one failure-path test that simulates git command failure and verifies the return value is "unknown".
• Keep all existing git/diff tests passing.

Extra context

getRepoRoot() is already tested in tests/git-diff.test.mjs; use that file’s temp-repo helper pattern for consistency.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.