optimise LatestMap.Merge by merging old into young

vendor/github.com/weaveworks/ps/map.go

@@ -42,6 +42,9 @@ type Map interface {
 	// This operation is O(log N) in the number of keys.
 	Lookup(key string) (interface{}, bool)
 
+	// First returns the "first" value in the map, if any, or nil.
+	First() interface{}
+
 	// Size returns the number of key value pairs in the map.
 	// This takes O(1) time.
 	Size() int
@@ -348,6 +351,10 @@ func lookupLowLevel(self *tree, partialHash, hash uint64) (interface{}, bool) {
 	return self.value, true
 }
 
+func (m *tree) First() interface{} {
+	return m.value
+}
+
 func (m *tree) Size() int {
 	return m.count
 }

vendor/manifest

@@ -1211,8 +1211,8 @@
 			"importpath": "github.com/weaveworks/ps",
 			"repository": "https://github.com/weaveworks/ps",
 			"vcs": "git",
-			"revision": "70d17b2d6f760345ef36a0748751eddf7290a92c",
-			"branch": "master",
+			"revision": "26c07ef9a67b5bd2f5707b9a1957e7492776d945",
+			"branch": "map-first",
 			"notests": true
 		},
 		{