usage: add set_warn_routine()

There are already set_die_routine() and set_error_routine(),
so let's add set_warn_routine() as this will be needed in a
following commit.

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

git-compat-util.h

@@ -440,6 +440,7 @@ static inline int const_error(void)
 
 extern void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list params));
 extern void set_error_routine(void (*routine)(const char *err, va_list params));
+extern void set_warn_routine(void (*routine)(const char *warn, va_list params));
 extern void set_die_is_recursing_routine(int (*routine)(void));
 extern void set_error_handle(FILE *);
 

usage.c

@@ -70,6 +70,11 @@ void set_error_routine(void (*routine)(const char *err, va_list params))
 	error_routine = routine;
 }
 
+void set_warn_routine(void (*routine)(const char *warn, va_list params))
+{
+	warn_routine = routine;
+}
+
 void set_die_is_recursing_routine(int (*routine)(void))
 {
 	die_is_recursing = routine;