[vim/vim] :diffget into an empty buffer creates an incorrect undo entry (Issue #20950)

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gnarendran created an issue (vim/vim#20950)

### Steps to reproduce

### Reproducer
* Create two files, one empty and another with two lines "1" and "2":
printf "" > temp1.txt
printf "1\n2\n" > temp2.txt

* Then open vim in diff mode:
vi -u NONE -d temp1.txt temp2.txt
In the empty buffer temp1.txt type:

do
u

do makes them them identical (correct)
u (undo) leaves a residual last line "2" (wrong).

* The issue is reproduced with any number of lines, with undo always leaving the residual last line. Reproduced in nvim nightly and vim 9.2, indicating it is present in the common diff code.



### Expected behaviour

After :undo or normal u, the buffer should be restored to its original empty state.


### Version of Vim

In Vim 9.2.390; Also reproduced in Neovim nightly.

### Environment

Fedora 43
Reproduced in all $TERM (xterm-256color, or xterm-ktty) and in gvim as well.
bash

### Logs and stack traces

```shell

```

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<div style=3D"display: flex; flex-wrap: wrap; white-space: pre-wrap; align-=
items: center; "><img height=3D"20" width=3D"20" style=3D"border-radius:50%=
; margin-right: 4px;" decoding=3D"async" src=3D"https://avatars.githubuserc=
ontent.com/u/23692893" /><strong>gnarendran</strong> created an issue <a hr=
ef=3D"https://github.com/vim/vim/issues/20950">(vim/vim#20950)</a></div>
<h3 dir=3D"auto">Steps to reproduce</h3>
<h3 dir=3D"auto">Reproducer</h3>
<ul dir=3D"auto">
<li>
<p dir=3D"auto">Create two files, one empty and another with two lines "1" =
and "2":<br>
printf "" &gt; temp1.txt<br>
printf "1\n2\n" &gt; temp2.txt</p>
</li>
<li>
<p dir=3D"auto">Then open vim in diff mode:<br>
vi -u NONE -d temp1.txt temp2.txt<br>
In the empty buffer temp1.txt type:</p>
</li>
</ul>
<p dir=3D"auto">do<br>
u</p>
<p dir=3D"auto">do makes them them identical (correct)<br>
u (undo) leaves a residual last line "2" (wrong).</p>
<ul dir=3D"auto">
<li>The issue is reproduced with any number of lines, with undo always leav=
ing the residual last line. Reproduced in nvim nightly and vim 9.2, indicat=
ing it is present in the common diff code.</li>
</ul>
<h3 dir=3D"auto">Expected behaviour</h3>
<p dir=3D"auto">After :undo or normal u, the buffer should be restored to i=
ts original empty state.</p>
<h3 dir=3D"auto">Version of Vim</h3>
<p dir=3D"auto">In Vim 9.2.390; Also reproduced in Neovim nightly.</p>
<h3 dir=3D"auto">Environment</h3>
<p dir=3D"auto">Fedora 43<br>
Reproduced in all $TERM (xterm-256color, or xterm-ktty) and in gvim as well=
.<br>
bash</p>
<h3 dir=3D"auto">Logs and stack traces</h3>
<div class=3D"highlight highlight-source-shell" dir=3D"auto"><pre class=3D"=
notranslate"></pre></div>

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