Problem: LspNotify autocmds were not being triggered for didChange
requests when being used during undo/redo (and possibly other) actions.
autocmds are blocked when calling on_lines() callbacks while doing the
undo/redo action.
Solution: Defer firing the autocmd until after the action is complete.
This is closer to what existed before, but now there's a check in the
deferred function to only fire the autocmd if the client is still
active and the buffer is still attached, if applicable.
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