Relocate buffer view after setting options that affect it (zyedidia#3743)

Whenever the user changes the value of an option that affects the
calculation of display params in updateDisplayInfo(), we should
immediately recalculate those params and relocate the buffer view
accordingly.

For example: after enabling ruler via `set ruler on`, if the cursor is
currently at the rightmost edge of the bufpane and softwrap is disabled,
then the cursor becomes invisible (since it is now outside the view,
since the buffer view width is decreased as a result of adding the ruler
but StartCol is not updated accordingly), it only becomes visible again
after the user types a character (or performs some other action that
triggers updateDisplayInfo() + relocate). Fix it.

Author: dmaluka

internal/display/bufwindow.go

@@ -61,6 +61,12 @@ func (w *BufWindow) SetBuffer(b *buffer.Buffer) {
 				c.LastWrappedVisualX = c.GetVisualX(true)
 			}
 		}
+
+		if option == "diffgutter" || option == "ruler" || option == "scrollbar" ||
+			option == "statusline" {
+			w.updateDisplayInfo()
+			w.Relocate()
+		}
 	}
 	b.GetVisualX = func(loc buffer.Loc) int {
 		return w.VLocFromLoc(loc).VisualX