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Problem:
With vim.treesitter.foldexpr, `o`-ing two lines above a folded region
opens the fold. This does not happen with legacy foldexprs. For example,
make a markdown file with the following text (without indentation),
enable treesitter fold, and follow the instruction in the text.
put cursor on this line and type zoo<Esc>
initially folded, revealed by zo
# then this fold will be opened
initially folded, revealed by o<Esc>
Analysis:
* `o` updates folds first (done in `changed_lines`), evaluating
foldexpr, and then invokes `on_bytes` (done in `extmark_splice`).
* Treesitter fold allocates the foldinfo for added lines (`add_range`)
on `on_bytes`.
* Therefore, when treesitter foldexpr is invoked while running `o`, it
sees outdated foldinfo.
Solution:
`extmark_splice`, and then `changed_lines`. This seems to be the
standard order in other places, e.g., `nvim_buf_set_lines`.