Merge pull request #2614 from k8s-infra-cherrypick-robot/cherry-pick-2529-to-release-0.15

[release-0.15] 🏃 Make client.MatchingLabels faster

Author: k8s-ci-robot

pkg/client/options.go

@@ -514,7 +514,8 @@ type MatchingLabels map[string]string
 func (m MatchingLabels) ApplyToList(opts *ListOptions) {
 	// TODO(directxman12): can we avoid reserializing this over and over?
 	if opts.LabelSelector == nil {
-		opts.LabelSelector = labels.NewSelector()
+		opts.LabelSelector = labels.SelectorFromValidatedSet(map[string]string(m))
+		return
 	}
 	// If there's already a selector, we need to AND the two together.
 	noValidSel := labels.SelectorFromValidatedSet(map[string]string(m))