enable shallow clone support in tests when necessary

tests/grafts/basic.c

@@ -7,6 +7,7 @@ static git_repository *g_repo;
 
 void test_grafts_basic__initialize(void)
 {
+	git_libgit2_opts(GIT_OPT_ENABLE_SHALLOW, 1);
 	g_repo = cl_git_sandbox_init("grafted.git");
 }
 

tests/grafts/shallow.c

@@ -19,6 +19,7 @@ void test_grafts_shallow__unset_feature_flag(void)
 
 void test_grafts_shallow__initialize(void)
 {
+	git_libgit2_opts(GIT_OPT_ENABLE_SHALLOW, 1);
 	cl_git_pass(git_oid_fromstr(&g_shallow_oid, "be3563ae3f795b2b4353bcce3a527ad0a4f7f644"));
 }