fix: ensure init/prepatch hooks are still repsected

this address a regression introduced in 984927a which
causes vue-router#1338 to resurface.

src/core/vdom/create-component.js

@@ -238,8 +238,21 @@ function installComponentHooks (data: VNodeData) {
   const hooks = data.hook || (data.hook = {})
   for (let i = 0; i < hooksToMerge.length; i++) {
     const key = hooksToMerge[i]
-    hooks[key] = componentVNodeHooks[key]
+    const existing = hooks[key]
+    const toMerge = componentVNodeHooks[key]
+    if (existing !== toMerge && !(existing && existing._merged)) {
+      hooks[key] = existing ? mergeHook(toMerge, existing) : toMerge
+    }
+  }
+}
+
+function mergeHook (f1, f2) {
+  const merged = (a, b, c, d) => {
+    f1(a, b, c, d)
+    f2(a, b, c, d)
   }
+  merged._merged = true
+  return merged
 }
 
 // transform component v-model info (value and callback) into

test/unit/modules/vdom/patch/edge-cases.spec.js

@@ -393,9 +393,15 @@ describe('vdom patch: edge cases', () => {
       extends: Base
     }
 
+    // sometimes we do need to tap into these internal hooks (e.g. in vue-router)
+    // so make sure it does work
+    const inlineHookSpy = jasmine.createSpy('inlineInit')
+
     const vm = new Vue({
       render (h) {
-        const data = { staticClass: 'text-red' }
+        const data = { staticClass: 'text-red', hook: {
+          init: inlineHookSpy
+        }}
 
         return h('div', [
           h(Foo, data),
@@ -405,5 +411,6 @@ describe('vdom patch: edge cases', () => {
     }).$mount()
 
     expect(vm.$el.textContent).toBe('FooBar')
+    expect(inlineHookSpy.calls.count()).toBe(2)
   })
 })