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
88617d1 (multi-pack-index: fix potential segfault without
sub-command, 2021-07-19).

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

Author: avar

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);

t/t5318-commit-graph.sh

@@ -6,10 +6,24 @@ test_description='commit graph'
 GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=0
 
 test_expect_success 'usage' '
-	test_expect_code 129 git commit-graph write blah &&
+	test_expect_code 129 git commit-graph write blah 2>err &&
 	test_expect_code 129 git commit-graph write verify
 '
 
+test_expect_success 'usage shown without sub-command' '
+	test_expect_code 129 git commit-graph 2>err &&
+	! grep error: err
+'
+
+test_expect_success 'usage shown with an error on unknown sub-command' '
+	cat >expect <<-\EOF &&
+	error: unrecognized subcommand: unknown
+	EOF
+	test_expect_code 129 git commit-graph unknown 2>stderr &&
+	grep error stderr >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'setup full repo' '
 	mkdir full &&
 	cd "$TRASH_DIRECTORY/full" &&