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/action/actions.go

@@ -1480,9 +1480,7 @@ func (h *BufPane) HalfPageDown() bool {
 func (h *BufPane) ToggleDiffGutter() bool {
 	if !h.Buf.Settings["diffgutter"].(bool) {
 		h.Buf.Settings["diffgutter"] = true
-		h.Buf.UpdateDiff(func(synchronous bool) {
-			screen.Redraw()
-		})
+		h.Buf.UpdateDiff()
 		InfoBar.Message("Enabled diff gutter")
 	} else {
 		h.Buf.Settings["diffgutter"] = false