object-file: stop using `the_hash_algo`

object-file: stop using `the_hash_algo`

There are a couple of users of the `the_hash_algo` macro, which
implicitly depends on `the_repository`. Adapt these callers to not do so
anymore, either by deriving it from already-available context or by
using `the_repository->hash_algo`. The latter variant doesn't yet help
to remove the global dependency, but such users will be adapted in the
following commits to not use `the_repository` anymore.

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