Previously, in `_mismatch()`, the `DefaultIterableDiffer` first checks
`_linkedRecords` for `itemTrackBy`, then checks `_unlinkedRecords`.
This cause the `DefaultIterableDiffer` to move "later" items that match the
`itemTrackBy` from the old collection, rather than using the "earlier" one.
Now we check `_unlinkedRecords` first, so that the `DefaultIterableDiffer`
can give a more stable and reasonable result after diffing. For example,
rather than (`a1` and `a2` have same trackById)
```
a1 b c a2 => b a2 c a1
```
we get
```
a1 b c a2 => b a1 c a2
```
where a1 and a2 retain their original order despite both
having the same track by value.
Fixes#23815
PR Close#23941
a8269264bf