Wrong patch when staged line by line, compared to other alternatives

**Describe the bug**
Wrong patch when line by line staging compared to using hunks or lazygit or ``git add -e``.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a empty repo
2. Stage & commit this on a file:
```rs
struct Earth;

impl Earth {
fn new() -> Self {
Self
}
}

struct Mars;

impl Mars {
fn new() -> Self {
Self
}
}
```
3. Update the file with this:
```rs
struct Earth;

impl Earth {
fn new() -> Self {
Self
}
fn into_mars(self) -> Mars {
Mars
}
}

struct Mars;

impl Mars {
fn new() -> Self {
Self
}
fn into_earth(self) -> Earth {
Earth
}
}
```
4. Use gitui to just stage ``into_mars`` and its body
5. Check the resulted patch with ``git diff --staged``

**Expected behavior**
I expect it to match other commands and tools.

**Screenshots**
(source code is different from the example above as I recorded these before creating the issue)
Using gitui: https://asciinema.org/a/dL8rEfpOHRQskpa7wmfVTJDQF
Using lazygit: https://asciinema.org/a/z0iSujDQzZO24wr7qXjKNwTf2

**Context (please complete the following information):**
- OS/Distro + Version: Windows 10 22H2
- GitUI Version: 0.21.0
- Rust version: 1.66.0-nightly

**Additional context**
I don't know if this is actually an issue because I don't know much about git patches; but it looks like it corrupts(?) the patch.

Inizia riproducendo passo per passo le fasi di staging in gitui 0.21.0 usando l'esempio Earth and Mars, quindi esamina il percorso di staging e confronta il risultato con git diff --staged, git add -e e lazygit. Il lavoro è completato quando la selezione del solo into_mars produce la stessa patch staged valida degli altri strumenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.