ignore findPrevVm check in coverage

Author: yyx990803

src/directives/repeat.js

@@ -687,6 +687,7 @@ module.exports = {
 
 function findPrevVm (vm, anchor, id) {
   var el = vm.$el.previousSibling
+  /* istanbul ignore if */
   if (!el) return
   while (
     (!el.__vue__ || el.__vue__.$options._repeatId !== id) &&