It is a design goal of extmarks that they allow precise tracking
of changes across undo/redo, including restore the exact positions
after a do/undo or undo/redo cycle. However this behavior is not useful
for all usecases. Many plugins won't keep marks around for long after
text changes, but uses them more like a cache until some external source
(like LSP semantic highlights) has fully updated to changed text and
then will explicitly readjust/replace extmarks as needed.
Add a "undo_restore" flag which is true by default (matches existing
behavior) but can be set to false to opt-out of this behavior.
Delete dead u_extmark_set() code.
68cb4a7405
