[vim/vim] cursorbind not resynchronized after undo changes diff correspondence (Issue #20982)

gnarendran created an issue (vim/vim#20982)

### Steps to reproduce

With `'cursorbind'` enabled in diff mode, undoing a change can leave the cursor in the other diff window at a stale, non-corresponding position.

This is particularly notable because `:help diffupdate` says that differences are kept updated after text changes and that `:diffupdate` can be used to update them. However, `:diffupdate` does not correct the stale cursor position.

[temp2.txt](https://github.com/user-attachments/files/30870669/temp2.txt)
[temp1.txt](https://github.com/user-attachments/files/30870668/temp1.txt)

`temp1.txt`:

```text
C11
C12
C31
C32
```

`temp2.txt`:

```text
C21
C22
A21
A22
C41
C42
```

Start:

```text
vim -u NONE -d "+set diffopt+=linematch:60" -d temp1.txt temp2.txt
```

At each stage, this command shows the line under the cursor in both windows:

```vim
echo printf("temp1.txt(%s) Vs temp2.txt(%s)", getline("."), trim(win_execute(win_getid(2), 'echo getline(".")')))
```

Initially:

```text
temp1.txt(C11) Vs temp2.txt(C21)
```

Execute `2dd` in the first window:

```text
temp1.txt(C31) Vs temp2.txt(C21)
```

This is valid: the deleted lines are represented by filler.

Execute `jk` in the first window:

```text
temp1.txt(C32) Vs temp2.txt(C41)
```

`cursorbind` has correctly moved the second cursor to the corresponding position.

Now execute `u` in the first window:

```text
temp1.txt(C11) Vs temp2.txt(C41)
```

This is *incorrect*: `C11` and `C41` do not correspond. The first cursor has moved because undo restored the deleted lines, but the cursor in the other diff window has not been resynchronized.

Executing `:diffupdate` does not correct the cursor. Moving the cursor vertically again causes `cursorbind` to resynchronize it.

Reproduced in both Vim and Neovim nightly.


### Expected behaviour

After undo changes the diff correspondence and moves the cursor to `C11`, and after the recommended :diffupdate, `cursorbind` should update the cursor in the other diff window to the corresponding position.

### Version of Vim

Vim 9.2.390

### Environment

Fedora 44
xterm
xterm-256color
bash

### Logs and stack traces

```shell

```

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