test-tool: use 'read-graph' helper

test-tool: use 'read-graph' helper

The 'git commit-graph read' subcommand is used in test scripts to check
that the commit-graph contents match the expected data. Mostly, this
helps check the header information and the list of chunks. Users do not
need this information, so move the functionality to a test helper.

Reported-by: Bryan Turner <redacted>
Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>