merge-ort: expand only for out-of-cone conflicts

Merge conflicts happen often enough to want to avoid expanding a sparse index when they happen, as long as those conflicts are within the sparse-checkout cone. If a conflict exists outside of the sparse-checkout cone, then we still need to expand before iterating over the index entries. This is critical to do in advance because of how the original_cache_nr is tracked to allow inserting and replacing cache entries. Iterate over the conflicted files and check if any paths are outside of the sparse-checkout cone. If so, then expand the full index. Add a test that demonstrates that we do not expand the index, even when we hit a conflict within the sparse-checkout cone. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>