t5318-commit-graph.sh: use core.commitGraph

The commit-graph tests should be checking that normal Git operations succeed and have matching output with and without the commit-graph feature enabled. However, the test was toggling 'core.graph' instead of the correct 'core.commitGraph' variable.

---
 t/t5318-commit-graph.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 77d85aefe7..59d0be2877 100755
@@ -28,8 +28,8 @@ test_expect_success 'create commits and repack' '
 '
 
 graph_git_two_modes() {
-	git -c core.graph=true $1 >output
-	git -c core.graph=false $1 >expect
+	git -c core.commitGraph=true $1 >output
+	git -c core.commitGraph=false $1 >expect
 	test_cmp output expect
 }
 

2.18.0.rc0