parse-options API: add a usage_msg_optf()

Add a usage_msg_optf() as a shorthand for the sort of
usage_msg_opt(xstrfmt(...)) used in builtin/stash.c. I'll make more
use of this function in builtin/cat-file.c shortly.

The disconnect between the "..." and "fmt" is a bit unusual, but it
works just fine and this keeps it consistent with usage_msg_opt(),
i.e. a caller of it can be moved to usage_msg_optf() and not have to
have its arguments re-arranged.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -1811,8 +1811,8 @@ int cmd_stash(int argc, const char **argv, const char *prefix)
 	else if (!strcmp(argv[0], "save"))
 		return !!save_stash(argc, argv, prefix);
 	else if (*argv[0] != '-')
-		usage_msg_opt(xstrfmt(_("unknown subcommand: %s"), argv[0]),
+		usage_msg_optf(_("unknown subcommand: %s"),
-			      git_stash_usage, options);
+			       git_stash_usage, options, argv[0]);
 
 	/* Assume 'stash push' */
 	strvec_push(&args, "push");

parse-options.c

@@ -1078,3 +1078,16 @@ void NORETURN usage_msg_opt(const char *msg,
 	fprintf(stderr, "fatal: %s\n\n", msg);
 	usage_with_options(usagestr, options);
 }
+
+void NORETURN usage_msg_optf(const char * const fmt,
+			     const char * const *usagestr,
+			     const struct option *options, ...)
+{
+	struct strbuf msg = STRBUF_INIT;
+	va_list ap;
+	va_start(ap, options);
+	strbuf_vaddf(&msg, fmt, ap);
+	va_end(ap);
+
+	usage_msg_opt(msg.buf, usagestr, options);
+}

parse-options.h

@@ -225,6 +225,16 @@ NORETURN void usage_msg_opt(const char *msg,
 			    const char * const *usagestr,
 			    const struct option *options);
 
+/**
+ * usage_msg_optf() is like usage_msg_opt() except that the first
+ * argument is a format string, and optional format arguments follow
+ * after the 3rd option.
+ */
+__attribute__((format (printf,1,4)))
+void NORETURN usage_msg_optf(const char *fmt,
+			     const char * const *usagestr,
+			     const struct option *options, ...);
+
 /*
  * Use these assertions for callbacks that expect to be called with NONEG and
  * NOARG respectively, and do not otherwise handle the "unset" and "arg"