Simplify UpdateDiff() interface

The callback passed to UpdateDiff() is superfluous: in the synchronous
case screen.Redraw() is not needed anyway (since the screen is redrawn
at every iteration of the main loop), and in the asynchronous case
UpdateDiff() can just call screen.Redraw() directly.

internal/display/bufwindow.go

@@ -384,18 +384,7 @@ func (w *BufWindow) displayBuffer() {
 
 	if b.ModifiedThisFrame {
 		if b.Settings["diffgutter"].(bool) {
-			b.UpdateDiff(func(synchronous bool) {
-				// If the diff was updated asynchronously, the outer call to
-				// displayBuffer might already be completed and we need to
-				// schedule a redraw in order to display the new diff.
-				// Note that this cannot lead to an infinite recursion
-				// because the modifications were cleared above so there won't
-				// be another call to UpdateDiff when displayBuffer is called
-				// during the redraw.
-				if !synchronous {
-					screen.Redraw()
-				}
-			})
+			b.UpdateDiff()
 		}
 		b.ModifiedThisFrame = false
 	}