fix(@angular-devkit/schematics): merge external schematics after execution

When an `externalSchematic` rule was used, the input tree was branched but never merged back into the input tree.  This resulted in a different tree instance being returned from the rule.  The `externalSchematic` rule now behaves like the `schematic` rule which merges the trees.

(cherry picked from commit 495459a)

Author: clydin

packages/angular_devkit/schematics/src/rules/schematic.ts

@@ -29,7 +29,14 @@ export function externalSchematic<OptionT extends object>(
     const collection = context.engine.createCollection(collectionName, context.schematic.collection);
     const schematic = collection.createSchematic(schematicName);
 
-    return schematic.call(options, observableOf(branch(input)), context, executionOptions);
+    return schematic.call(options, observableOf(branch(input)), context, executionOptions).pipe(
+      last(),
+      map(x => {
+        input.merge(x, MergeStrategy.AllowOverwriteConflict);
+
+        return input;
+      }),
+    );
   };
 }