hash-ll: merge with "hash.h"

hash-ll: merge with "hash.h"

The "hash-ll.h" header was introduced via d1cbe1e6d8 (hash-ll.h: split
out of hash.h to remove dependency on repository.h, 2023-04-22) to make
explicit the split between hash-related functions that rely on the
global `the_repository`, and those that don't. This split is no longer
necessary now that we we have removed the reliance on `the_repository`.

Merge "hash-ll.h" back into "hash.h". This causes some code units to not
include "repository.h" anymore, which requires us to add some forward
declarations.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>