t1092: add tests for status/add and sparse files

Before moving to update 'git status' and 'git add' to work with sparse indexes, add an explicit test that ensures the sparse-index works the same as a normal sparse-checkout when the worktree contains directories and files outside of the sparse cone. Specifically, 'folder1/a' is a file in our test repo, but 'folder1' is not in the sparse cone. When 'folder1/a' is modified, the file is not shown as modified and adding it will fail. This is new behavior as of a20f704 (add: warn when asked to update SKIP_WORKTREE entries, 2021-04-08). Before that change, these adds would be silently ignored. Untracked files are fine: adding new files both with 'git add .' and 'git add folder1/' works just as in a full checkout. This may not be entirely desirable, but we are not intending to change behavior at the moment, only document it. A future change could alter the behavior to be more sensible, and this test could be modified to satisfy the new expected behavior. Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>