Clear cursors before performing undo or redo

This fix isn't ideal because it removes the cursors but it does work

Fixes #798

cmd/micro/buffer.go

@@ -562,3 +562,12 @@ func (b *Buffer) ClearMatches() {
 		b.SetState(i, nil)
 	}
 }
+
+func (b *Buffer) clearCursors() {
+	for i := 1; i < len(b.cursors); i++ {
+		b.cursors[i] = nil
+	}
+	b.cursors = b.cursors[:1]
+	b.UpdateCursors()
+	b.Cursor.ResetSelection()
+}