fix: correctly attribute AI lines in merge commit stats

[svarlamov]

Summary

Fixes #910. When AI resolves a merge conflict, git ai blame correctly attributes lines to the AI, but git ai stats head was showing 100% human / 0% AI. Three root causes:

1. accepted_lines_from_attestations() early-returned (0, empty) for all merge commits, unconditionally skipping attestation matching.
2. stats_for_commit_stats() set added_lines_by_file to an empty HashMap for merge commits, so there were no lines to match against attestations even if the early return were removed.
3. git show --numstat uses combined-diff format for merge commits, which only shows files differing from ALL parents (conflict resolutions), missing cleanly-merged changes. Replaced with git diff commit^1 commit --numstat for merge commits.

What changed

• Removed the is_merge_commit parameter and early return from accepted_lines_from_attestations()
• added_lines_by_file is now always computed by diffing against the first parent (works for both regular and merge commits)
• New get_git_diff_stats_first_parent() helper uses git diff commit^1..commit --numstat
• Extracted parse_numstat_output() to share parsing logic between the two numstat paths
• Two new TestRepo integration tests (conflicting + non-conflicting merge with AI checkpoint)
• Updated existing unit tests to reflect corrected behavior

Review & Testing Checklist for Human

• Double-counting in range stats: Merge commits now report git_diff_added_lines > 0 for all changes vs first parent. If git ai stats <range> sums across both the feature branch commits AND the merge commit, the same lines could be counted twice. Verify how range stats aggregation works and whether this is an issue.
• Octopus merges: The fix uses commit^1 (first parent). Verify this is correct for octopus merges (3+ parents) — should it always be the first parent?
• Behavioral change for merge commits without AI notes: Previously, merge commits with no authorship log showed human_additions=0, git_diff_added_lines=0. Now they show the actual first-parent diff (e.g., human_additions=1, git_diff_added_lines=1). Confirm this is the desired behavior.
• Manual verification: On a real repo with AI-resolved merge conflicts (like the one in issue AI-resolved merge conflict results in human stats, but blame is correct #910), run git ai stats head and confirm the output now shows AI attribution correctly.

Notes

• The integration tests simulate the real-world flow: git merge --no-commit -X theirs → git ai checkpoint mock_ai <file> → stage_all_and_commit. This mirrors what happens when an AI tool resolves conflicts and git-ai's checkpoint mechanism tracks it.
• All 2562 integration tests + 1249 unit tests pass. Lint and format checks are clean.

Link to Devin session: https://app.devin.ai/sessions/6bd8df4f03d94a51a75575a7769f9b4f
Requested by: @svarlamov

---

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.