dulwich broken with sparse checkout

[dtrifiro]

When using a git repo with sparse-checkout set (git sparse-checkout set), this sets the worktreeconfig = true in the git config. This is currently not supported by dulwich <= 0.21.2, but should be in 0.21.3, see jelmer/dulwich#1133

Original discuss context: https://discuss.dvc.org/t/dvc-with-git-sparse-checkout/1480

[pmrowla]

This is a known issue, sparse checkout is also unsupported in libgit2. Worktree support is also separate from sparse checkouts and it will likely still be broken even after the dulwich change. (The dulwich change really only adds support for git worktree https://git-scm.com/docs/git-worktree)

Sparse checkout is a non-trivial git feature for us to support

jelmer/dulwich#405
libgit2/libgit2#2263