pack-bitmap-write: stop depending on `the_repository`

pack-bitmap-write: stop depending on `the_repository`

There are multiple sites in "pack-bitmap-write.c" where we use the
global `the_repository` variable, either explicitly or implicitly by
using `the_hash_algo`.

Refactor the code so that the `struct bitmap_writer` stores the
repository it is getting initialized with. Like this, we can adapt
callsites that use `the_repository` to instead use the repository
provided by the writer.

Remove the `USE_THE_REPOSITORY_VARIABLE` define.

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