t/helper/test-read-graph.c: prepare repo settings

t/helper/test-read-graph.c: prepare repo settings

The read-graph test-tool is used by a number of the commit-graph test to
assert various properties about a commit-graph. Previously, this program
never ran 'prepare_repo_settings()'. There was no need to do so, since
none of the commit-graph machinery is affected by the repo settings.

In the next patch, the commit-graph machinery's behavior will become
dependent on the repo settings, and so loading them before running the
rest of the test tool is critical.

As such, teach the test tool to call 'prepare_repo_settings()'.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>