Skipping setting the language on the git index text buffer improves
snappiness for (very) large git diff multibuffers and also significantly
lower peak and steady-state RSS. When tested on the chromium repo with
`git reset HEAD~1000` and *all* language extensions installed we notice:
* current `main`: peak RSS ~45GB, ss RSS ~39GB
* this patch: peak RSS ~30GB, ss RSS ~27GB

Release Notes:

- Improved memory usage of large git diff multibuffers.