t/helper/test-read-graph.c: support commit-graph chains

In 61df89c (commit-graph: don't early exit(1) on e.g. "git status",
2019-03-25), the former 'load_commit_graph_one' was refactored into
'open_commit_graph' and 'load_commit_graph_one_fd_st' as a means of
avoiding an early-exit from non-library code.

However, 'load_commit_graph_one' does not support commit-graph chains,
and hence the 'read-graph' test tool does not work with them.

Replace 'load_commit_graph_one' with 'read_commit_graph_one' in order to
support commit-graph chains. In the spirit of 61df89c,
'read_commit_graph_one' does not ever 'die()', making it a suitable
replacement here.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

t/helper/test-read-graph.c

@@ -7,26 +7,15 @@
 int cmd__read_graph(int argc, const char **argv)
 {
 	struct commit_graph *graph = NULL;
-	char *graph_name;
-	int open_ok;
-	int fd;
-	struct stat st;
 	struct object_directory *odb;
 
 	setup_git_directory();
 	odb = the_repository->objects->odb;
 
-	graph_name = get_commit_graph_filename(odb);
-
-	open_ok = open_commit_graph(graph_name, &fd, &st);
-	if (!open_ok)
-		die_errno(_("Could not open commit-graph '%s'"), graph_name);
-
-	graph = load_commit_graph_one_fd_st(fd, &st, odb);
+	graph = read_commit_graph_one(the_repository, odb);
 	if (!graph)
 		return 1;
 
-	FREE_AND_NULL(graph_name);
 
 	printf("header: %08x %d %d %d %d\n",
 		ntohl(*(uint32_t*)graph->data),