Staging lines doesn’t stage

**Describe the bug**
Pressing `s` or any mapped key on `diff_stage_lines` doesn’t stage anything. However, it does unstage.

**To Reproduce**
Steps to reproduce the behavior:
1. Just try to stage some lines.

**Expected behavior**
It should stage.

**Screenshots**
N/A

**Context (please complete the following information):**

```
- OS/Distro + Version: Linux awake 6.1.12-arch1-1 #1 SMP PREEMPT_DYNAMIC Tue, 14 Feb 2023 22:08:08 +0000 x86_64 GNU/Linux
- GitUI Version: `gitui 0.22.1`
- Rust version: `rustc 1.66.1 (90743e729 2023-01-10)`
```

**Additional context**
N/A