import: git sparse-checkout

[casperdcl]

dvc import https://some/git/repo/ some_file should probably do a sparse checkout.

Currently looks like it does a full clone:

https://github.com/iterative/dvc/blob/05cc023164766fbbec91642d7d8af513c8d4079b/dvc/stage.py#L941 would call https://github.com/iterative/dvc/blob/05cc023164766fbbec91642d7d8af513c8d4079b/dvc/dependency/repo.py#L73-L81

On a related note, is the repo locally cached by default https://github.com/iterative/dvc/blob/05cc023164766fbbec91642d7d8af513c8d4079b/dvc/dependency/repo.py#L78-L79 so that dvc update won't re-clone?

[shcheklein]

@Suor you should have more info about his I think?

[efiop]

Related to #3473

[Suor]

Local caches only used within one dvc execution or one python session.

Sparse checkout or at least shallow checkouts might be used, this will complicate out external_repo() abstraction, including its caching. But looks like there is no way around.

[casperdcl]

I think we had a discussion with @efiop and @shcheklein about which is more important to fix first (persistent repo cache or sparse checkout) and there was no clear outcome. Will open a new issue for persistent repo cache...