fix: fix BadMutable plugin bug in .diff
fix #1022
**Describe the bug**
When I use RelativeTime plugin with BadMutable plugin in my react project, I get wrong answer.

For Example: The time variable gets `in infinity years` instead of `4 months ago`.
```javascript
import React from 'react';
import dayjs from 'dayjs';
import relativeTime from 'dayjs/plugin/relativeTime';
import badMutable from 'dayjs'/plugin/badMutable';

dayjs.extend(relativeTime);
dayjs.extend(badMutable);

const App = () => {
    // 1588262400000 stands for 2020-05-1
    const time = dayjs(new Date(1588262400000)).fromNow();
    return <div>{time}</div>
}
```

**Expected behavior**
As above, the time variable should be `4 months ago`.

**Information**
 - Day.js Version: 1.8.34
 - OS: MAC
 - Browser: Chrome 83
 - Time zone: 2020-08-22 China Beijing

RelativeTime plugin bug while using BadMutable plugin
diff --git a/src/utils.js b/src/utils.js
index ae81679f8..324bc5e11 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -18,9 +18,9 @@ const monthDiff = (a, b) => {
   // function from moment.js in order to keep the same result
   if (a.date() < b.date()) return -monthDiff(b, a)
   const wholeMonthDiff = ((b.year() - a.year()) * 12) + (b.month() - a.month())
-  const anchor = a.add(wholeMonthDiff, C.M)
+  const anchor = a.clone().add(wholeMonthDiff, C.M)
   const c = b - anchor < 0
-  const anchor2 = a.add(wholeMonthDiff + (c ? -1 : 1), C.M)
+  const anchor2 = a.clone().add(wholeMonthDiff + (c ? -1 : 1), C.M)
   return +(-(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) :
     (anchor2 - anchor)))) || 0)
 }

diff --git a/test/plugin/badMutable.test.js b/test/plugin/badMutable.test.js
index fb4dc723e..bfd351520 100644
--- a/test/plugin/badMutable.test.js
+++ b/test/plugin/badMutable.test.js
@@ -164,6 +164,16 @@ it('Locale', () => {
   expect(d.format(format)).toBe(m.format(format))
 })
 
+it('Diff', () => {
+  const d = dayjs()
+  const m = moment()
+  const unit = 'year'
+  const d2 = d.clone().add(1, unit)
+  const m2 = m.clone().add(1, unit)
+  expect(d.diff(d2, unit)).toBe(-1)
+  expect(m.diff(m2, unit)).toBe(-1)
+})
+
 it('isAfter isBefore isSame', () => {
   const d = dayjs()
   const format = dayjs().format()

{"test/plugin/badMutable.test.js": {"run": "PASS", "test": "FAIL", "fix": "PASS"}}
{"test/timezone.test.js:Add Time days (DST)": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/get-set.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/utc.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/quarterOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/objectSupport.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/fi.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sk.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sv.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/advancedFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/toArray.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:Utc Offset": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isSameOrBefore.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/pluralGetSet.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:Diff (DST)": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/utc-utcOffset.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/keys.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/utils.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/relativeTime.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/parse.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isMoment.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isYesterday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/pl.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/toObject.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/localizedFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/updateLocale.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/constructor.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isSameOrAfter.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/comparison.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isLeapYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/query.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/localeData.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/et.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/calendar.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isTomorrow.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/zh.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isToday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/declarations.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/customParseFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/ru.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isoWeek.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/duration.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/minMax.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/dayOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isoWeeksInYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:UTC add day in DST": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/manipulate.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/display.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/buddhistEra.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/uk.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/cs.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/zh-cn.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/en.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isBetween.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}}
{"test/plugin/badMutable.test.js": {"run": "PASS", "test": "FAIL", "fix": "PASS"}}
{"passed_count": 60, "failed_count": 1, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/get-set.test.js", "test/plugin.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/objectSupport.test.js", "test/locale/fi.test.js", "test/locale/sk.test.js", "test/locale/sv.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/weekYear.test.js", "test/plugin/toArray.test.js", "test/timezone.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/isSameOrBefore.test.js", "test/plugin/pluralGetSet.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/keys.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/plugin/isMoment.test.js", "test/plugin/isYesterday.test.js", "test/locale/pl.test.js", "test/plugin/toObject.test.js", "test/plugin/localizedFormat.test.js", "test/plugin/updateLocale.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/comparison.test.js", "test/plugin/isLeapYear.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/locale/et.test.js", "test/plugin/calendar.test.js", "test/plugin/isTomorrow.test.js", "test/locale/zh.test.js", "test/plugin/isToday.test.js", "test/plugin/declarations.test.js", "test/locale.test.js", "test/plugin/badMutable.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/isoWeek.test.js", "test/plugin/duration.test.js", "test/plugin/minMax.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/manipulate.test.js", "test/display.test.js", "test/plugin/buddhistEra.test.js", "test/locale/uk.test.js", "test/locale/cs.test.js", "test/locale/zh-cn.test.js", "test/locale/en.test.js", "test/plugin/isBetween.test.js"], "failed_tests": ["test/plugin/timezone.test.js"], "skipped_tests": []}
{"passed_count": 59, "failed_count": 2, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/plugin.test.js", "test/get-set.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/objectSupport.test.js", "test/locale/fi.test.js", "test/locale/sk.test.js", "test/locale/sv.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/toArray.test.js", "test/plugin/weekYear.test.js", "test/timezone.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/isSameOrBefore.test.js", "test/plugin/pluralGetSet.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/keys.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/plugin/isMoment.test.js", "test/plugin/isYesterday.test.js", "test/locale/pl.test.js", "test/plugin/toObject.test.js", "test/plugin/localizedFormat.test.js", "test/plugin/updateLocale.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/comparison.test.js", "test/plugin/isLeapYear.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/plugin/isTomorrow.test.js", "test/plugin/calendar.test.js", "test/locale/et.test.js", "test/locale/zh.test.js", "test/plugin/isToday.test.js", "test/plugin/declarations.test.js", "test/locale.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/isoWeek.test.js", "test/plugin/duration.test.js", "test/plugin/minMax.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/plugin/buddhistEra.test.js", "test/display.test.js", "test/locale/cs.test.js", "test/locale/uk.test.js", "test/manipulate.test.js", "test/locale/zh-cn.test.js", "test/locale/en.test.js", "test/plugin/isBetween.test.js"], "failed_tests": ["test/plugin/timezone.test.js", "test/plugin/badMutable.test.js"], "skipped_tests": []}
{"passed_count": 60, "failed_count": 1, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/plugin.test.js", "test/get-set.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/objectSupport.test.js", "test/locale/fi.test.js", "test/locale/sk.test.js", "test/locale/sv.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/weekYear.test.js", "test/plugin/toArray.test.js", "test/timezone.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/isSameOrBefore.test.js", "test/plugin/pluralGetSet.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/keys.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/plugin/isMoment.test.js", "test/plugin/isYesterday.test.js", "test/locale/pl.test.js", "test/plugin/toObject.test.js", "test/plugin/localizedFormat.test.js", "test/plugin/updateLocale.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/plugin/isLeapYear.test.js", "test/comparison.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/locale/et.test.js", "test/plugin/isTomorrow.test.js", "test/plugin/calendar.test.js", "test/locale/zh.test.js", "test/plugin/isToday.test.js", "test/plugin/declarations.test.js", "test/plugin/badMutable.test.js", "test/locale.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/isoWeek.test.js", "test/plugin/duration.test.js", "test/plugin/minMax.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/manipulate.test.js", "test/display.test.js", "test/plugin/buddhistEra.test.js", "test/locale/cs.test.js", "test/locale/uk.test.js", "test/locale/zh-cn.test.js", "test/locale/en.test.js", "test/plugin/isBetween.test.js"], "failed_tests": ["test/plugin/timezone.test.js"], "skipped_tests": []}