code: func (old *Manifest_CIPDPackage) Diff(new *Manifest_CIPDPackage) ManifestDiff_Stat {
  	return zeroCmpTwo(old, new, func() ManifestDiff_Stat {
  		// Version and PackagePattern don't matter for diff purposes.
  		return zeroCmpTwo(old.InstanceId, new.InstanceId, nil)
  	})
  }
docs: // Diff generates a ManifestDiff_Stat reflecting the difference between the `old`
  // CIPDPackage and the `new` one.