mv: rename check_dir_in_index() to empty_dir_has_sparse_contents()

mv: rename check_dir_in_index() to empty_dir_has_sparse_contents()

Method check_dir_in_index() introduced in b91a2b6594 (mv: add
check_dir_in_index() and solve general dir check issue, 2022-06-30)
does not describe its intent and behavior well.

Change its name to empty_dir_has_sparse_contents(), which more
precisely describes its purpose.

Reverse the return values, check_dir_in_index() return 0 for success
and 1 for failure; reverse the values so empty_dir_has_sparse_contents()
return 1 for success and 0 for failure. These values are more intuitive
because 1 usually means "has" and 0 means "not found".

Also modify the documentation to better align with the method's
intent and behavior.

Helped-by: Derrick Stolee <derrickstolee@github.com>
Helped-by: Victoria Dye <vdye@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>