commit-graph: show "unexpected subcommand" error

Bring the "commit-graph" command in line with the error output and
general pattern in cmd_multi_pack_index().

Let's test for that output, and also cover the same potential bug as
was fixed in the multi-pack-index command in
88617d11f9 (multi-pack-index: fix potential segfault without
sub-command, 2021-07-19).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit-graph.c

@@ -345,6 +345,7 @@ int cmd_commit_graph(int argc, const char **argv, const char *prefix)
 	else if (argc && !strcmp(argv[0], "write"))
 		return graph_write(argc, argv);
 
+	error(_("unrecognized subcommand: %s"), argv[0]);
 usage:
 	usage_with_options(builtin_commit_graph_usage,
 			   builtin_commit_graph_options);