t3705: test that 'sparse_entry' is unstaged

t3705: test that 'sparse_entry' is unstaged

The tests in t3705-add-sparse-checkout.sh check to see how 'git add'
behaves with paths outside the sparse-checkout definition. These
currently check to see if a given warning is present but not that the
index is not updated with the sparse entries. Add a new
'test_sparse_entry_unstaged' helper to be sure 'git add' is behaving
correctly.

We need to modify setup_sparse_entry to actually commit the sparse_entry
file so it exists at HEAD and as an entry in the index, but its exact
contents are not staged in the index.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>