date-picker: fix week highlight on year boundary (#13883)

When the selected week spans across two years (e.g. 2018-12-30 2019-01-05),
they should be highlighted in both month tables.

packages/date-picker/src/basic/date-table.vue

@@ -32,7 +32,7 @@
 </template>
 
 <script>
-  import { getFirstDayOfMonth, getDayCountOfMonth, getWeekNumber, getStartDateOfMonth, nextDate, isDate, clearTime as _clearTime} from '../util';
+  import { getFirstDayOfMonth, getDayCountOfMonth, getWeekNumber, getStartDateOfMonth, prevDate, nextDate, isDate, clearTime as _clearTime} from '../util';
   import Locale from 'element-ui/src/mixins/locale';
   import { arrayFindIndex, arrayFind, coerceTruthyValueToArray } from 'element-ui/src/utils/util';
 
@@ -321,8 +321,12 @@
 
         newDate.setDate(parseInt(cell.text, 10));
 
-        const valueYear = isDate(this.value) ? this.value.getFullYear() : null;
-        return year === valueYear && getWeekNumber(newDate) === getWeekNumber(this.value);
+        if (isDate(this.value)) {
+          const dayOffset = (this.value.getDay() - this.firstDayOfWeek + 7) % 7 - 1;
+          const weekDate = prevDate(this.value, dayOffset);
+          return weekDate.getTime() === newDate.getTime();
+        }
+        return false;
       },
 
       markRange(minDate, maxDate) {