cache_name_compare(): do not truncate while comparing paths

cache_name_compare(): do not truncate while comparing paths

We failed to use ce_namelen() equivalent and instead only compared
up to the CE_NAMEMASK bytes by mistake.  Adding an overlong path
that shares the same common prefix as an existing entry in the index
did not add a new entry, but instead replaced the existing one, as
the result.

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