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>
diff --git a/builtin/commit.c b/builtin/commit.c
index ef5e622..c84062b 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1559,8 +1559,8 @@ struct repository *repo UNUSED)
 		OPT_END(),
 	};
 
-	if (argc == 2 && !strcmp(argv[1], "-h"))
-		usage_with_options(builtin_status_usage, builtin_status_options);
+	show_usage_with_options_if_asked(argc, argv,
+					 builtin_status_usage, builtin_status_options);
 
 	prepare_repo_settings(the_repository);
 	the_repository->settings.command_requires_full_index = 0;
@@ -1736,8 +1736,8 @@ int cmd_commit(int argc,
 	struct strbuf err = STRBUF_INIT;
 	int ret = 0;
 
-	if (argc == 2 && !strcmp(argv[1], "-h"))
-		usage_with_options(builtin_commit_usage, builtin_commit_options);
+	show_usage_with_options_if_asked(argc, argv,
+					 builtin_commit_usage, builtin_commit_options);
 
 	prepare_repo_settings(the_repository);
 	the_repository->settings.command_requires_full_index = 0;