* Collect on_bytes and flush at the invocation of the scheduled callback
to take account of commands that triggers multiple on_bytes.
* More accurately track movement of folds so that foldexpr returns
reasonable values even when the scheduled computation is not run yet.
* Start computing folds from the line above (+ foldminlines) the changed
lines to handle the folds that are removed due to the size limit.
* Shrink folds that end at the line at which another fold starts to
assign proper level to that line.
* Use level '=' for lines that are not computed yet.
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