dir.c: use the cache_* macro to access the current index

dir.c: use the cache_* macro to access the current index

These codepaths always start from the_index and use index_*
functions, but there is no reason to do so.  Use the compatibility
cache_* macro to access the current in-core index like everybody
else.

While at it, fix typo in the comment for a function to check if a
path within a directory appears in the index.

Signed-off-by: Junio C Hamano <gitster@pobox.com>