Problem: `vim.diagnostic.get_prev()` / `vim.diagnostic.get_next()` use
logical diagnostic positions and consider extmark validity.
`vim.diagnostic.get()` only uses original positions and doesn't care
about extmark validity. This results in inconsistency between these
APIs.
Solution: use original positions in `vim.diagnostic.get_prev()` and
`vim.diagnostic.get_next()` and don't consider extmark validity to match
previous behavior, which is consistent with `vim.diagnostic.get`.
bc4bddbb21
