Preserve line selection relations after discarding modified line(s)

### The feature request

Currently, after discarding a modified line or a block of lines, the relationship between the line selection and the modified lines is not always preserved. The current behavior updates the line selection by truncating it from the bottom which will result in all unselected lines below the discard being out of sync with their corresponding modified line.

I propose that line selection and modified line relations are always preserved after discards.

### Proposed solution

The current behavior provides no benefit to the user and will waste their time when they need to circumvent or fix its results.

### Additional context

https://github.com/desktop/desktop/assets/33699564/9f87e813-a0d5-4717-bc12-a09b95d334ff