Do not modify the manifest during dep ensure -update

Author: carolynvs

cmd/dep/ensure.go

@@ -149,11 +149,13 @@ func (cmd *ensureCommand) Run(ctx *dep.Ctx, args []string) error {
 
 	sw := dep.SafeWriter{
 		Root:          p.AbsRoot,
-		Manifest:      p.Manifest,
 		Lock:          p.Lock,
 		NewLock:       solution,
 		SourceManager: sm,
 	}
+	if !cmd.update {
+		sw.Manifest = p.Manifest
+	}
 
 	// check if vendor exists, because if the locks are the same but
 	// vendor does not exist we should write vendor

cmd/dep/ensure_test.go

@@ -252,18 +252,10 @@ func TryToDoSomething() error {
 	h.Cd(h.Path("src/thing"))
 
 	h.Run("ensure", "-update", "github.com/carolynvs/go-dep-test")
-	
-	expectedManifest := `{
-    "dependencies": {
-        "github.com/carolynvs/go-dep-test": {
-            "version": "^0.1.0"
-        }
-    }
-}
-`
+
 	manifest := h.ReadManifest()
-	if manifest != expectedManifest {
-		t.Fatalf("expected '%s', got '%s'", expectedManifest, manifest)
+	if manifest != origManifest {
+		t.Fatalf("The manifest should not be modified during an update. Expected %s, got %s", origManifest, manifest)
 	}
 
 	expectedLock := `{