Make "index_name_exists()" return the cache_entry it found

Make "index_name_exists()" return the cache_entry it found

This allows verify_absent() in unpack_trees() to use the hash chains
rather than looking it up using the binary search.

Perhaps more importantly, it's also going to be useful for the next phase,
where we actually start looking at the cache entry when we do
case-insensitive lookups and checking the result.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>