Diff includes merge-commit noise from main, inflating file count

[benthomasson]

Problem

When a feature branch has been rebased or contains merge commits from main, git diff main...feature-branch includes all those merged files in the diff. This causes the review to process hundreds of files that are not part of the actual feature change.

Example

MR !340 (feature/runtime-model-fallback) on agents-python:

• git diff --stat main...feature/runtime-model-fallback shows 377 files changed, 41,829 insertions
• git diff --stat dc8f17b98^..a79d3f913 (feature commits only) shows 9 files changed, 750 insertions

The 377/41k number is merge noise. The review should only see the 9/750.

Suggested Fix

Use git merge-base main feature-branch to find the true fork point, or allow the user to specify a commit range (--from-commit). GitLab MRs expose diff_refs.base_sha which gives the correct base.

Alternatively, detect when the diff is dominated by merge commits and warn the user.

Related

This caused issue #18 -- the inflated diff was what made the review stall.

[benthomasson]

Update on root cause: The issue is that the feature branch was forked from an older main and never rebased. So main...feature-branch includes all commits that landed on main after the fork point, even though they have nothing to do with the MR. This is normal git behavior, not a bug in the diff algorithm.

The suggestion still stands: code-review could detect when the diff is dominated by commits that exist on both branches (i.e., the branch hasn't been rebased recently) and either warn the user or allow a --from-commit flag to scope the review to the actual feature commits.