cache-tree: wrap the_index based wrappers with #ifdef

cache-tree: wrap the_index based wrappers with #ifdef

This puts update_main_cache_tree() and write_cache_as_tree() in the
same group of "index compat" functions that assume the_index
implicitly, which should only be used within builtin/ or t/helper.

sequencer.c is also updated to not use these functions. As of now, no
files outside builtin/ use these functions anymore.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>