Move name hashing functions into a file of its own

Move name hashing functions into a file of its own

It's really totally separate functionality, and if we want to start
doing case-insensitive hash lookups, I'd rather do it when it's
separated out.

It also renames "remove_index_entry()" to "remove_name_hash()", because
that really describes the thing better. It doesn't actually remove the
index entry, that's done by "remove_index_entry_at()", which is something
very different, despite the similarity in names.

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