update-index: reduce scope of index expansion in do_reupdate

update-index: reduce scope of index expansion in do_reupdate

Replace unconditional index expansion in 'do_reupdate()' with one scoped to
only where a full index is needed. A full index is only required in
'do_reupdate()' when a sparse directory in the index differs from HEAD; in
that case, the index is expanded and the operation restarted.

Because the index should only be expanded if a sparse directory is modified,
add a test ensuring the index is not expanded when differences only exist
within the sparse cone.

Signed-off-by: Victoria Dye <vdye@github.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>