player: change interaction between revert_seek and ab-loops

The result isn't quite what I imagined, because the A-point is never
marked as a seek point (so you can't jump between A and B), but it's
still slightly better than before.

player/command.c

@@ -4627,6 +4627,7 @@ static void command_event(struct MPContext *mpctx, int event, void *arg)
                 ctx->prev_pts < opts->ab_loop[1] &&
                 now >= opts->ab_loop[1])
             {
+                mark_seek(mpctx);
                 queue_seek(mpctx, MPSEEK_ABSOLUTE, opts->ab_loop[0], 1, false);
             }
         }