The previous implementation fetched ALL PR refs with:
git fetch origin +refs/pull/*:refs/remotes/origin/pull/*
This is extremely slow for repos with many PRs (700+ PRs in unity-builder).
Now fetches only the specific PR ref needed, e.g., for pull/731/merge:
git fetch origin +refs/pull/731/merge:... +refs/pull/731/head:...
This should significantly speed up the Cloud Runner integrity tests.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>