PERF: Recover `changes_applied` performance (#31698)

#30985 caused `object.save` performance regression since calling
`changes` in `changes_applied` is very slow.
We don't need to call the expensive method in `changes_applied` as long
as `@attributes` is tracked by mutation tracker.

https://gist.github.com/kamipo/1a9f4f3891803b914fc72ede98268aa2

Before:

```
Warming up --------------------------------------
create_string_columns
                        73.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        722.256  (± 5.8%) i/s -      3.650k in   5.073031s
```

After:

```
Warming up --------------------------------------
create_string_columns
                        96.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        950.224  (± 7.7%) i/s -      4.800k in   5.084837s
```

0 files changed, 0 insertions, 0 deletions