[Feature Request] Discard staged only

**Is your feature request related to a problem? Please describe.**

I am in a situation where I need the staged and keep the unstaged changes unstaged.

We may also need to do this with a single file that is partially staged.

**Describe the solution you'd like**

1. In the reset menu options which opens by pressing D, we should find an option to discard staged changes only.
2. It is reasonable also to have this option for the discard changes for one file menu, which opened by pressing d

**Describe alternatives you've considered**

```
patch -p1 -R < <(git diff --cached) && git reset
```

**Additional context**