usage: add show_usage_if_asked()

Some commands call usage() when they are asked to give the help
message with "git cmd -h", but this has the same problem as we
fixed with callers of usage_with_options() for the same purpose.

Introduce a helper function that captures the common pattern

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

and replaces it with

	show_usage_if_asked(argc, argv, usage);

to help correct these 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.

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

git-compat-util.h

@@ -701,6 +701,8 @@ int error_errno(const char *err, ...) __attribute__((format (printf, 1, 2)));
 void warning(const char *err, ...) __attribute__((format (printf, 1, 2)));
 void warning_errno(const char *err, ...) __attribute__((format (printf, 1, 2)));
 
+void show_usage_if_asked(int ac, const char **av, const char *err);
+
 #ifndef NO_OPENSSL
 #ifdef APPLE_COMMON_CRYPTO
 #include "compat/apple-common-crypto.h"