config API: use get_error_routine(), not vreportf()

Change the git_die_config() function added in 5a80e97c82 (config: add
`git_die_config()` to the config-set API, 2014-08-07) to use the
public callbacks in the usage.[ch] API instead of the the underlying
vreportf() function.

In preceding commits the rest of the vreportf() users outside of
usage.c was migrated to die_message(), so we can now make it "static".

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

config.c

@@ -2555,11 +2555,12 @@ void git_die_config(const char *key, const char *err, ...)
 {
 	const struct string_list *values;
 	struct key_value_info *kv_info;
+	report_fn error_fn = get_error_routine();
 
 	if (err) {
 		va_list params;
 		va_start(params, err);
-		vreportf("error: ", err, params);
+		error_fn(err, params);
 		va_end(params);
 	}
 	values = git_config_get_value_multi(key);

git-compat-util.h

@@ -474,7 +474,6 @@ static inline int git_has_dir_sep(const char *path)
 struct strbuf;
 
 /* General helper functions */
-void vreportf(const char *prefix, const char *err, va_list params);
 NORETURN void usage(const char *err);
 NORETURN void usagef(const char *err, ...) __attribute__((format (printf, 1, 2)));
 NORETURN void die(const char *err, ...) __attribute__((format (printf, 1, 2)));

usage.c

@@ -6,7 +6,7 @@
 #include "git-compat-util.h"
 #include "cache.h"
 
-void vreportf(const char *prefix, const char *err, va_list params)
+static void vreportf(const char *prefix, const char *err, va_list params)
 {
 	char msg[4096];
 	char *p, *pend = msg + sizeof(msg);