Fixes a crash when copying and pasting a mixed empty/nonempty
multi-cursor selection.

`Editor::do_copy` is meant to insert a newline after each non-final
selection that doesn't copy the entire line, but
`prev_selection_was_entire_line` is currently based on the current
selection instead of the previous one. `Editor::cut_common` does already
correctly track this state across loop iterations.

This can lead to an out of bounds crash when pasting because the
indexing logic in `Editor::do_paste` relies on these newlines being
added correctly, depending on `is_entire_line`.

Repro:

```
foo
bar
```
1. select `foo`
2. option-click after `bar` to add an empty selection
```
«fooˇ»
barˇ
```
3. copy
4. paste

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Fixed a crash when copying and pasting using multiple cursors.