name-hash: add index_dir_find()

name-hash: add index_dir_find()

index_dir_exists() returns a boolean to indicate if there is a
case-insensitive match in the directory name-hash, but does not
provide the caller with the exact spelling of that match.

Create index_dir_find() to do the case-insensitive search *and*
optionally return the spelling of the matched directory prefix in a
provided strbuf.

To avoid code duplication, convert index_dir_exists() to be a trivial
wrapper around the new index_dir_find().

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>