parse-options: add show_usage_with_options_if_asked()

Many commands call usage_with_options() when they are asked to give
the help message, but it sends the help text to the standard error
stream.  When the user asked for it with "git cmd -h", the help
message is the primary output from the command, hence we should send
it to the standard output stream, instead.

Introduce a helper function that captures the common pattern

	if (argc == 2 && !strcmp(argv[1], "-h"))
		usage_with_options(usage, options);

and replaces it with

	show_usage_with_options_if_asked(argc, argv, usage, options);

to help correct code paths.

Note that this helper function still exits with status 129, and
t0012 insists on it.  After converting all the mistaken callers of
usage_with_options() to call this new helper, we may want to address
it---the end user is asking us to give the help text, and we are
doing exactly as asked, so there is no reason to exit with non-zero
status.

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

parse-options.c

@@ -1282,6 +1282,16 @@ void NORETURN usage_with_options(const char * const *usagestr,
 	exit(129);
 }
 
+void show_usage_with_options_if_asked(int ac, const char **av,
+				      const char * const *usagestr,
+				      const struct option *opts)
+{
+	if (ac == 2 && !strcmp(av[1], "-h")) {
+		usage_with_options_internal(NULL, usagestr, opts, 0, 0);
+		exit(129);
+	}
+}
+
 void NORETURN usage_msg_opt(const char *msg,
 		   const char * const *usagestr,
 		   const struct option *options)

parse-options.h

@@ -402,6 +402,10 @@ int parse_options(int argc, const char **argv, const char *prefix,
 NORETURN void usage_with_options(const char * const *usagestr,
 				 const struct option *options);
 
+void show_usage_with_options_if_asked(int ac, const char **av,
+				      const char * const *usage,
+				      const struct option *options);
+
 NORETURN void usage_msg_opt(const char *msg,
 			    const char * const *usagestr,
 			    const struct option *options);