builtins: send usage_with_options() help text to standard output

Using the show_usage_with_options_if_asked() helper we introduced
earlier, fix callers of usage_with_options() that want to show the
help text when explicitly asked by the end-user.  The help text now
goes to the standard output stream for them.

The test in t7600 for "git merge -h" may want to be retired, as the
same is covered by t0012 already, but it is specifically testing that
the "-h" option gets a response even with a corrupt index file, so
for now let's leave it there.

Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout-index.c

@@ -250,9 +250,9 @@ int cmd_checkout_index(int argc,
 		OPT_END()
 	};
 
-	if (argc == 2 && !strcmp(argv[1], "-h"))
-		usage_with_options(builtin_checkout_index_usage,
-				   builtin_checkout_index_options);
+	show_usage_with_options_if_asked(argc, argv,
+					 builtin_checkout_index_usage,
+					 builtin_checkout_index_options);
 	git_config(git_default_config, NULL);
 	prefix_length = prefix ? strlen(prefix) : 0;