parse-options: add new function "usage_msg_opt"

This function can be used instead of "usage_with_options" when you
want to print an error message before the usage string.

It may be useful because:

if (condition)
	usage_msg_opt("condition is false", usage, opts);

is shorter than:

if (condition) {
	fprintf(stderr, "condition is false\n\n");
	usage_with_options(usage, opts);
}

and may be more consistent.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-replace.c

@@ -121,7 +121,7 @@ int cmd_replace(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
-	argc = parse_options(argc, argv, options, git_replace_usage, 0);
+	argc = parse_options(argc, argv, prefix, options, git_replace_usage, 0);
 
 	if (list && delete)
 		usage_with_options(git_replace_usage, options);

parse-options.c

@@ -555,6 +555,14 @@ void usage_with_options(const char * const *usagestr,
 	exit(129);
 }
 
+void usage_msg_opt(const char *msg,
+		   const char * const *usagestr,
+		   const struct option *options)
+{
+	fprintf(stderr, "%s\n\n", msg);
+	usage_with_options(usagestr, options);
+}
+
 int parse_options_usage(const char * const *usagestr,
 			const struct option *opts)
 {

parse-options.h

@@ -132,6 +132,10 @@ extern int parse_options(int argc, const char **argv, const char *prefix,
 extern NORETURN void usage_with_options(const char * const *usagestr,
                                         const struct option *options);
 
+extern NORETURN void usage_msg_opt(const char *msg,
+				   const char * const *usagestr,
+				   const struct option *options);
+
 /*----- incremental advanced APIs -----*/
 
 enum {