fix(runtime-core/scheduler): only allow watch callbacks to be self-triggering

fix #1740

Previous fix for #1727 caused `watchEffect` to also recursively trigger
itself on reactive array mutations which implicitly registers array
`.length` as dependencies and mutates it at the same time.

This fix limits recursive trigger behavior to only `watch()` callbacks
since code inside the callback do not register dependencies and
mutations are always explicitly intended.

packages/runtime-core/src/apiWatch.ts

@@ -7,7 +7,7 @@ import {
   ReactiveEffectOptions,
   isReactive
 } from '@vue/reactivity'
-import { queueJob } from './scheduler'
+import { queueJob, SchedulerJob } from './scheduler'
 import {
   EMPTY_OBJ,
   isObject,
@@ -232,7 +232,7 @@ function doWatch(
   }
 
   let oldValue = isArray(source) ? [] : INITIAL_WATCHER_VALUE
-  const job = () => {
+  const job: SchedulerJob = () => {
     if (!runner.active) {
       return
     }
@@ -258,6 +258,10 @@ function doWatch(
     }
   }
 
+  // important: mark the job as a watcher callback so that scheduler knows it
+  // it is allowed to self-trigger (#1727)
+  job.cb = !!cb
+
   let scheduler: (job: () => any) => void
   if (flush === 'sync') {
     scheduler = job