Read base_text from the `MultiBufferSnapshot` instead of the live
`BufferDiff` entity in `prepare_restore_change`. The live entity's
base_text may already reflect a newer git HEAD while the snapshot still
holds old hunk byte ranges, causing `Rope::slice` to panic.

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 is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes ZED-64P

Release Notes:

- N/A