fix: only compare PRs targeting the same base branch

[zkoppert]

Why

Stacked PRs (where PR b targets branch a and PR a targets main) are incorrectly flagged as conflicting because they naturally touch the same files. However, they cannot conflict at merge time since they target different branches.

Fixes #78

Impact

In large monorepos where this is being used, 8-11% of open PRs target a non-default branch. These PRs are candidates for false-positive conflict reports under the old logic. This fix eliminates that entire class of false positives.

What changed

Modified find_file_overlaps() in conflict_detector.py to index PRs by (base_branch, filename) instead of just filename. This ensures only PRs targeting the same base branch are compared for overlapping line changes.

Before vs After

Scenario	Before	After
PR b → branch a, PR a → main (stacked)	❌ False conflict reported	✅ No conflict (different base branches)
Two PRs both → main touching same lines	✅ Conflict reported	✅ Conflict reported (unchanged)
PRs targeting different release branches	❌ Possible false conflict	✅ No conflict (different base branches)

Tradeoffs

Chosen approach: Index by (base_branch, filename) tuple in the file overlap detection.

Alternative considered: Explicit stacked-PR detection by checking if one PR's head branch equals another PR's base branch. Rejected because: (1) it's redundant after the base-branch equality guard, and (2) branch-name equality isn't a reliable model for all stacking workflows.

Testing

• 4 new unit tests covering: different base branches, stacked PRs, same-base-branch still conflicts, and mixed scenarios
• All 273 tests pass with 99% code coverage
• Manual integration tests with DRY_RUN=true:
  • github/docs (74 PRs, 1 non-main target) — completed, 2 conflicts detected
  • a private UI monorepo (679 PRs, 75 stacked PRs) — completed, 461 conflicts detected after same-author filtering
  • In both repos, stacked PRs targeting non-default branches were correctly excluded from cross-base-branch comparisons and produced zero false positives
• Deterministic verification script confirmed: PRs with different base branches are never compared, while PRs targeting the same base branch are still correctly flagged

Rollout

Low-risk change — the logic only adds a tighter filter to an existing comparison. PRs that previously conflicted correctly will continue to conflict (same base branch). The only behavioral change is eliminating false positives for cross-base-branch comparisons.

What to watch after merge: If users report missed conflicts, verify whether the involved PRs target the same base branch. The fix intentionally skips cross-base-branch comparisons.

[Copilot]

Pull request overview

This PR fixes false-positive conflict detection for stacked PRs by ensuring overlap checks only compare pull requests that target the same base branch.

Changes:

• Update find_file_overlaps() to index by (base_branch, filename) rather than only filename.
• Extend unit tests to cover different-base-branch and stacked-PR scenarios.
• Loosen pylint’s max-module-lines limit to accommodate the expanded test module.

Show a summary per file

File	Description
conflict_detector.py	Restricts file/line overlap comparisons to PRs sharing the same base branch.
test_conflict_detector.py	Adds coverage for cross-base-branch and stacked-PR cases; extends PR test helper to allow base/head branches.
.github/linters/.python-lint	Increases allowed Python module length to avoid lint failures with the enlarged test file.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 2