t5318: extract helpers to lib-commit-graph.sh

t5318: extract helpers to lib-commit-graph.sh

The graph_git_behavior helper is useful for testing that certain Git
commands behave the same when using the commit-graph and when not using
the commit-graph. Extract it to a new lib-commit-graph.sh file for use
in new test scripts that will split out from t5318.

While doing this extraction, also extract graph_read_expect and the
logic for priming the test_oid_cache.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>