sparse-index: prevent repo root from becoming sparse

Prevent the repository root from being collapsed into a sparse directory by treating an empty path as "inside the sparse-checkout". When collapsing a sparse index (e.g. in 'git sparse-checkout reapply'), the root directory typically could not become a sparse directory due to the presence of in-cone root-level files and directories. However, if no such in-cone files or directories were present, there was no explicit check signaling that the "repository root path" (an empty string, in the case of 'convert_to_sparse(...)') was in-cone, and a sparse directory index entry would be created from the repository root directory. The documentation in Documentation/git-sparse-checkout.txt explicitly states that the files in the root directory are expected to be in-cone for a cone-mode sparse-checkout. Collapsing the root into a sparse directory entry violates that assumption, as sparse directory entries are expected to be outside the sparse cone and have SKIP_WORKTREE enabled. This invalid state in turn causes issues with commands that interact with the index, e.g. 'git status'. Treating an empty (root) path as in-cone prevents the creation of a root sparse directory in 'convert_to_sparse(...)'. Because the repository root is otherwise never compared with sparse patterns (in both cone-mode and non-cone sparse-checkouts), the new check does not cause additional changes to how sparse patterns are applied. Helped-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Victoria Dye <vdye@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>