Fix #13937: Correctly scope .finish() following multi-element .animate()

cc @gnarf37

Size reduction suggestions welcome :(

Compared to 1.x-master @ 28db00fb6f35dcc9acfe61ed4f89e75f72a5199e
```
   raw     gz
  +217    +85 dist/jquery.js
   +39    +20 dist/jquery.min.js
```