Merge branch 'ds/commit-graph-tests'

We can now optionally run tests with commit-graph enabled.

* ds/commit-graph-tests:
  commit-graph: define GIT_TEST_COMMIT_GRAPH

commit-graph.h

@@ -6,6 +6,8 @@
 #include "string-list.h"
 #include "cache.h"
 
+#define GIT_TEST_COMMIT_GRAPH "GIT_TEST_COMMIT_GRAPH"
+
 struct commit;
 
 char *get_commit_graph_filename(const char *obj_dir);