Problem: When using treesitter foldexpr,
* :diffput/get open diff folds, and
* folds are not updated in other windows that contain the updated
buffer.
Solution: Update folds in all windows that contain the updated buffer
and use expr foldmethod.
Problem: Treesitter fold is not updated if treesitter hightlight is not
active. More precisely, updating folds requires `LanguageTree:parse()`.
Solution: Call `parse()` before computing folds and compute folds when
lines are added/removed.
This doesn't guarantee correctness of the folds, because some changes
that don't add/remove line won't update the folds even if they should
(e.g. adding pair of braces). But it is good enough for most cases,
while not introducing big overhead.
Also, if highlighting is active, it is likely that
`TSHighlighter._on_buf` already ran `parse()` (or vice versa).
