Introduce usagef() that takes a printf-style format

Some new callers would want to use printf-like formatting, when issuing
their usage messages.  An option is to change usage() itself also be like
printf(), which would make it similar to die() and warn().

But usage() is typically fixed, as opposed to die() and warn() that gives
diagnostics depending on the situation.  Indeed, the majority of strings
given by existing callsites to usage() are fixed strings.  If we were to
make usage() take printf-style format, they all need to be changed to have
"%s" as their first argument.

So instead, introduce usagef() so that limited number of callers can use
it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -189,6 +189,7 @@ extern char *gitbasename(char *);
 
 /* General helper functions */
 extern NORETURN void usage(const char *err);
+extern NORETURN void usagef(const char *err, ...) __attribute__((format (printf, 1, 2)));
 extern NORETURN void die(const char *err, ...) __attribute__((format (printf, 1, 2)));
 extern NORETURN void die_errno(const char *err, ...) __attribute__((format (printf, 1, 2)));
 extern int error(const char *err, ...) __attribute__((format (printf, 1, 2)));

usage.c

@@ -12,9 +12,9 @@ static void report(const char *prefix, const char *err, va_list params)
 	fprintf(stderr, "%s%s\n", prefix, msg);
 }
 
-static NORETURN void usage_builtin(const char *err)
+static NORETURN void usage_builtin(const char *err, va_list params)
 {
-	fprintf(stderr, "usage: %s\n", err);
+	report("usage: ", err, params);
 	exit(129);
 }
 
@@ -36,7 +36,7 @@ static void warn_builtin(const char *warn, va_list params)
 
 /* If we are in a dlopen()ed .so write to a global variable would segfault
  * (ugh), so keep things static. */
-static NORETURN_PTR void (*usage_routine)(const char *err) = usage_builtin;
+static NORETURN_PTR void (*usage_routine)(const char *err, va_list params) = usage_builtin;
 static NORETURN_PTR void (*die_routine)(const char *err, va_list params) = die_builtin;
 static void (*error_routine)(const char *err, va_list params) = error_builtin;
 static void (*warn_routine)(const char *err, va_list params) = warn_builtin;
@@ -46,9 +46,18 @@ void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list param
 	die_routine = routine;
 }
 
+void usagef(const char *err, ...)
+{
+	va_list params;
+
+	va_start(params, err);
+	usage_routine(err, params);
+	va_end(params);
+}
+
 void usage(const char *err)
 {
-	usage_routine(err);
+	usagef("%s", err);
 }
 
 void die(const char *err, ...)