commit-graph: stop using `the_repository`

commit-graph: stop using `the_repository`

There's still a bunch of uses of `the_repository` in "commit-graph.c",
which we want to stop using due to it being a global variable. Refactor
the code to stop using `the_repository` in favor of the repository
provided via the calling context.

This allows us to drop the `USE_THE_REPOSITORY_VARIABLE` macro.

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