Simplify UpdateDiff() interface: remove unneeded callback (since UpdateDiff() can simply call screen.Redraw() directly in the async case, and doesn't need to call it at all in the sync case).
Add linearray locking in async diff update, to fix potential race when the buffer is modified in the meantime (just like in other similar cases fixed recently in https://github.com/zyedidia/micro/pull/3224).
UpdateDiff()interface: remove unneeded callback (sinceUpdateDiff()can simply callscreen.Redraw()directly in the async case, and doesn't need to call it at all in the sync case).