fix: propagate fatal git errors from hasCommits

[404-Page-Found]

Fixes #306

hasCommits() now treats only an unborn symbolic HEAD with no stored branch ref as the legitimate zero-commit case. Fatal Git failures (including detached or corrupt HEAD/refs) are rethrown with Git stderr preserved.

Adds a regression test that corrupts the current branch ref and verifies the Git failure is surfaced instead of being reported as an empty repository.

Validation attempted locally, but the execution environment cannot resolve github.com. CI should run the full format and test matrix.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 65718e4 and 881298f.

📒 Files selected for processing (2)

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

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[cubic-dev-ai]

All reported issues were addressed

<sub>Shadow auto-approve: would not auto-approve because issues were found.

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 2 files

<sub>Shadow auto-approve: would not auto-approve because issues were found.

Re-trigger cubic</sub>

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cubic-dev-ai]

0 issues found across 2 files (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Shadow auto-approve: would require human review. This PR changes hasCommits() to propagate fatal Git errors unless the repo is unborn; a human should review the error propagation implications.

Re-trigger cubic</sub>