🤖 Fix code review: eliminate inverse deltas when branch is behind base

[ammar-agent]

Problem

When a branch is behind origin/main and the "Uncommitted" checkbox is enabled in Code Review, inverse deltas (deletions) appear for commits that landed on origin/main after branching.

Root cause: Toggling "Uncommitted" switched between different git diff modes:

• git diff origin/main...HEAD (three-dot, stable merge-base) when uncommitted=false
• git diff origin/main (two-dot, tip comparison) when uncommitted=true

The two-dot diff compares the tip of origin/main to the working directory, showing commits Y, Z, W as deletions since they exist on main but not in the feature branch.

Solution

Use explicit git merge-base when includeUncommitted is true:

# includeUncommitted=false (committed only)
git diff origin/main...HEAD

# includeUncommitted=true (committed + uncommitted)
git diff $(git merge-base origin/main HEAD)

Benefits:

• Stable merge-base: Doesn't change when base ref moves forward
• Single unified diff: Avoids duplicate hunks from concatenation
• Clear semantics: "Uncommitted" literally means "include working directory changes"

Test Coverage

Added comprehensive test: "should not show inverse deltas when branch is behind base ref"

• Creates feature branch from main
• Adds 3 commits to main after branching (simulates branch falling behind)
• Adds committed + uncommitted changes to feature branch
• Verifies: Shows only feature changes, NOT inverse deltas from main

All 662 tests pass ✓

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