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 <bturner@atlassian.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Makefile b/Makefile
index 58b92af..44997f6 100644
--- a/Makefile
+++ b/Makefile
@@ -727,6 +727,7 @@
 TEST_BUILTINS_OBJS += test-progress.o
 TEST_BUILTINS_OBJS += test-reach.o
 TEST_BUILTINS_OBJS += test-read-cache.o
+TEST_BUILTINS_OBJS += test-read-graph.o
 TEST_BUILTINS_OBJS += test-read-midx.o
 TEST_BUILTINS_OBJS += test-ref-store.o
 TEST_BUILTINS_OBJS += test-regex.o