remote: checkout: reuse _checkout_file functionality

[pared]

• Have you followed the guidelines in our
  Contributing document?

• Does your PR affect documented changes or does it add new functionality
  that should be documented? If yes, have you created a PR for
  dvc.org documenting it or at
  least opened an issue for it? If so, please add a link to it.

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Fixes #2444

[efiop]

Thanks! Please don't forget to post the results of the benchmark here when you have them.