Add structured-merge-diff vendor

This is where most of the logic actually lives.

vendor/sigs.k8s.io/structured-merge-diff/fieldpath/managers.go

@@ -0,0 +1,73 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+    http://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package fieldpath
+
+// APIVersion describes the version of an object or of a fieldset.
+type APIVersion string
+
+// VersionedSet associates a version to a set.
+type VersionedSet struct {
+	*Set
+	APIVersion APIVersion
+}
+
+// ManagedFields is a map from manager to VersionedSet (what they own in
+// what version).
+type ManagedFields map[string]*VersionedSet
+
+// Difference returns a symmetric difference between two Managers. If a
+// given user's entry has version X in lhs and version Y in rhs, then
+// the return value for that user will be from rhs. If the difference for
+// a user is an empty set, that user will not be inserted in the map.
+func (lhs ManagedFields) Difference(rhs ManagedFields) ManagedFields {
+	diff := ManagedFields{}
+
+	for manager, left := range lhs {
+		right, ok := rhs[manager]
+		if !ok {
+			if !left.Empty() {
+				diff[manager] = left
+			}
+			continue
+		}
+
+		// If we have sets in both but their version
+		// differs, we don't even diff and keep the
+		// entire thing.
+		if left.APIVersion != right.APIVersion {
+			diff[manager] = right
+			continue
+		}
+
+		newSet := left.Difference(right.Set).Union(right.Difference(left.Set))
+		if !newSet.Empty() {
+			diff[manager] = &VersionedSet{
+				Set:        newSet,
+				APIVersion: right.APIVersion,
+			}
+		}
+	}
+
+	for manager, set := range rhs {
+		if _, ok := lhs[manager]; ok {
+			// Already done
+			continue
+		}
+		if !set.Empty() {
+			diff[manager] = set
+		}
+	}
+
+	return diff
+}