Introduce `USE_THE_REPOSITORY_VARIABLE` macro

Use of the the_repository variable is nowadays considered to be deprecated, and over time we want to convert our codebase to stop using it in favor of explicitly passing down the repository to functions via parameters. This effort faces some important problems though.

• It is hard to prove that a certain code unit does not use the_repository anymore when sending patch series. The reviewer has no way to verify that it's not used anymore without reading through the code itself.

• It is easy to sneak in new usages of the_repository by accident into a code unit that is already the_repository-clean.

• There are many functions which implicitly use the_repository, which is really hard to spot.

This patch series aims to address those problems by introducing a new USE_THE_REPOSITORY_VARIABLE macro. When unset, then the declarations of the_repository, the_hash_algo and some functions that implicitly depend on them will be hidden away. This makes it trivial to demonstrate that a code unit is the_repository-free by removing the definition of any such macro.

The patch series doesn't aim to be perfect. There are still many functions that implicitly depend on the_repository which are not hidden away. Those can be addressed over time, either by guarding them as required or, even better, removing such functions altogether.

The series is structured as follows:

• Patches 1 to 8 refactor "hash.h" such that its functions do not depend on the_repository anymore. As these are used almost everywhere, the whole patch series would be rather moot without such a refactoring as everything would depend on the_repository.

• Patch 9 introduces USE_THE_REPOSITORY_VARIABLE, guarding the declaration of the_repository and the_hash_algo.

• Patch 11 merges "hash-ll.h" back into "hash.h" as the split is now mostly pointless.

• Patches 12 to 20 refactor various users of "hex.h" to not use functions depending on the_repository anymore.

• Patch 21 guards declarations of functions that depend on the_repository in "hex.h".

The series depends on ps/ref-sotrage-migration at 25a0023f (builtin/refs: new command to migrate ref storage formats, 2024-06-06). This is moslty due to patch 10, which fixes a cyclic include that breaks the build once we merge "hash-ll.h" back into "hash.h".