multi-pack-index: refactor "goto usage" pattern

Refactor the "goto usage" pattern added in
cd57bc4 (builtin/multi-pack-index.c: display usage on unrecognized
command, 2021-03-30) and 88617d1 (multi-pack-index: fix potential
segfault without sub-command, 2021-07-19) to maintain the same
brevity, but in a form that doesn't run afoul of the recommendation in
CodingGuidelines about braces:

    When there are multiple arms to a conditional and some of them
    require braces, enclose even a single line block in braces for
    consistency[...]

Let's also change "argv == 0" to juts "!argv", per:

    Do not explicitly compare an integral value with constant 0 or
    '\0', or a pointer value with constant NULL[...]

I'm changing this because in a subsequent commit I'll make
builtin/commit-graph.c use the same pattern, having the two similarly
structured commands match aids readability.

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/multi-pack-index.c

@@ -164,7 +164,7 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	if (!opts.object_dir)
 		opts.object_dir = get_object_directory();
 
-	if (argc == 0)
+	if (!argc)
 		goto usage;
 
 	if (!strcmp(argv[0], "repack"))
@@ -175,10 +175,9 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return cmd_multi_pack_index_verify(argc, argv);
 	else if (!strcmp(argv[0], "expire"))
 		return cmd_multi_pack_index_expire(argc, argv);
-	else {
-		error(_("unrecognized subcommand: %s"), argv[0]);
+
+	error(_("unrecognized subcommand: %s"), argv[0]);
 usage:
-		usage_with_options(builtin_multi_pack_index_usage,
-				   builtin_multi_pack_index_options);
-	}
+	usage_with_options(builtin_multi_pack_index_usage,
+			   builtin_multi_pack_index_options);
 }