import/update: cache git repos/clones

[casperdcl]

dvc import https://some/git/repo/ some_file
dvc update  # should not re-clone, should only pull into existing cache

• related: import: git sparse-checkout #3438, 'dvc get' is cloning the full bitbucket repo when only one branch is needed #3473

[Suor]

The thing is cache is not persisted between dvc runs, if we make it persist then that won't reclone only make git pull in dvc update.

[casperdcl]

yes; this is about making it persistent & pulling rather than re-cloning.

[jorgeorpinel]

What about a repo cache at the user level? Could be a system config var so you can disable it, like analytics.

Context: #4203

[casperdcl]

in light of #4246 being merged going to downgrade priority here...