t/helper/test-repository: celebrate independence from the_repository

t/helper/test-repository: celebrate independence from the_repository

dade47c06c (commit-graph: add repo arg to graph readers, 2018-07-11)
brought more independence from the_repository to the commit graph, however
it was not completely independent of the_repository, as the previous
patches show.

To ensure we're not accessing the_repository by accident, we'd ideally
assign NULL to the_repository to trigger a segfault on access.

We currently have a temporary hack in cache.h, which relies on
the_hash_algo (which is a short form of the_repository->hash_algo) to
be set, so we cannot do that. The next best thing is to set all fields of
the_repository to 0, so any accidental access is more likely to be found.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>