oss-fuzz/commit-graph: set up hash algorithm

oss-fuzz/commit-graph: set up hash algorithm

Our fuzzing setups don't work in a proper repository, but only use the
in-memory configured `the_repository`. Consequently, we never go through
the full repository setup procedures and thus do not set up the hash
algo used by the repository.

The commit-graph fuzzer does rely on a properly initialized hash algo
though. Initialize it explicitly.

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