add: update --chmod to skip sparse paths

add: update --chmod to skip sparse paths

We added checks for path_in_sparse_checkout() to portions of 'git add'
that add warnings and prevent staging a modification, but we skipped the
--chmod mode. Update chmod_pathspec() to ignore cache entries whose path
is outside of the sparse-checkout cone (unless --sparse is provided).
Add a test in t3705.

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