Use batch API for Download, fallback to legacy if unsupported

[rubyist]

The smudge filters still used the direct legacy api via the Download() function. This PR provides a new Download() which uses the batch API and falls back to a DownloadLegacy() if the server does not support the batch API.

This fixes #708

[rubyist]

I think there's still plenty to refactor around this code, but we might want to hold off on that until we completely remove support of the legacy API. Then we can just remove all the legacy cruft at once.

[sinbad]

👍
Yeah it's slightly confusing at the moment because DownloadCheck() also uses the old API but then it's only ever called by the individualApiRoutine, so the use of the batch version & fallback is higher up. Download() is only ever used by the smudge filter now though so this does address the core issue for now, so I think it's still worth doing ahead of a more general tidy up.

[technoweenie]

I updated this so that lfs.Download() only attempts batch transfers if lfs.batch is not false.

so I think it's still worth doing ahead of a more general tidy up.

We absolutely have to do this before starting on your SSH pet project :)

[rubyist]

👍 to your changes