pack-revindex: stop depending on `the_repository`

pack-revindex: stop depending on `the_repository`

There are multiple sites in "pack-revindex.c" where we use the global
`the_repository` variable, either explicitly or implicitly by using
`the_hash_algo`. In all of those cases we already have a repository
available in the calling context though.

Refactor the code to instead use the caller-provided repository and
remove the `USE_THE_REPOSITORY_VARIABLE` define.

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