t/helper: fix leaking commit graph in "read-graph" subcommand

t/helper: fix leaking commit graph in "read-graph" subcommand

We're leaking the commit-graph in the "test-helper read-graph"
subcommand, but as the leak is annotated with `UNLEAK()` the leak
sanitizer doesn't complain.

Fix the leak by calling `free_commit_graph()`. Besides getting rid of
the `UNLEAK()` annotation, it also increases code coverage because we
properly release resources as Git would do it, as well.

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