We need to reparse the base text using the same language as the main
buffer, whenever the main buffer's language changed. We already do this
for the uncommitted diff, but it was missing for the branch (OID-based)
diffs. (We don't need it for the unstaged diff because we don't
currently show you that base text anywhere in the UI, so its language is
immaterial.)

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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A