isSame with itself is false during DST

**Describe the bug**
Comparing a date with itself (`d.isSame(d)`) returns false if the date is during daylight saving time.

```
// Running node with TZ='Europe/Zurich'

d = dayjs.tz("2021-10-01 00:00:00", "Europe/Athens")
d.isSame(d) // returns false
```

A test project to reproduce the bug can be found at https://github.com/faberchri/dayjs_bugs

The bug does not occur if node runs with `TZ='UTC'`.

**Expected behavior**
Comparing a date to itself with `isSame` should always return true.

**Information**
- Day.js Version 1.10.7
- OS: macOS 11.6.1
- node v16.13.0
- Time zone: "Europe/Zurich"

Start with the linked dayjs_bugs reproduction project and run the example under TZ='Europe/Zurich', comparing it with TZ='UTC'. Trace the isSame behavior for the Europe/Athens date; done means d.isSame(d) returns true during daylight saving time without regressing the UTC case.

Written by the indexing model from the issue text.